In World of Warcraft, as in almost every other MMOG, you have to team up with others in order to get the best items in the game. This means that people who don't raid, and people who don't play as much as others, are unable to gear up their characters in the same way as those who raid a lot. This of course leads to a discussion of whether or not it is fair that non-raiders and casuals are unable to develop their characters in the same manner as the hardcore raiders. Some people claim it is not fair that some people are privileged based purely on their choice of "play-style" - everybody is paying the same monthly fee after all.
Most players will look on the reward/investment ratio, where investment is part risk and part effort. It is not unreasonable that low risk, high effort encounters give equivalent rewards to a high risk, low effort encounter. WoW does this to some extent with world drops; these are epics that drop off regular mobs with very low drop chance. But, unlike the epics that drop in a raid, these are not "predictable" - and predictability is important when you're trying to acquire specific items. The faction rewards work better, where vendors will give you access to new items based on your popularity with their faction.
Now, if one joins a raid, there is the issue of loot distribution. Most raids are guild run, and most guilds use either DKP (Dragon Kill Points), or have raid leaders who distribute the loot as they see fit. Here's another obstacle for the non-raider or casual player. DKP is a reward people get for participating in raids; these points are in turn used to acquire items. People who participate in more raids get more DKP, and thus first dibs on the items they want. Less active people have to wait in queue. A lot of people don't like DKP systems, yet a majority of the large guilds use DKP in some form, why is this?
The issues people have with DKP are quite similar to the issues they have with the distribution of epic items between those who raid and those who don't. Mainly, I guess, because it is the same people who benefit most in both cases.
I'm not a big fan of DKP systems personally; I think they are fundamentally flawed. They do, however, serve their purpose most of the time. My guild uses a very simple DKP system. We basically give points depending on how important or difficult an activity is for the guild. This can lead to some strange situations, e.g. while we were learning the Nefarian encounter we gave almost as much DKP for wiping on Nefarian as we did for a full Molten Core run. Due to our system being very simple we can change the DKP reward as the risk goes down or increase it to motivate the guild to do things we consider beneficial to the guild.
Now, what does this have to do with non-raiders not getting epics? Well, what if we think of epics as Blizzards DKP system? What if we think of epics as not only the reward for a certain investment - but also a carrot to encourage certain behavior? In which case, what would be desired behavior, and why?
World of Warcraft is a game, however, it is also a product from which Blizzard (and more importantly Vivendi) want to profit. All available data suggest that players who are members of an active guild generally stay with the game, and thus pay their subscription, for far longer than those who are not. I can only speculate, but chances are that Blizzard want to encourage people join guilds, groups, and raids, get to know others, and thus stay with the game longer - and by staying they give other players a reason to keep playing as well. Not only does this increase the profit from each player, there's also Metcalf's law to consider.
When non-raiders say: "I pay the same subscription fee as those who raid - I should be able to get epics as well!" it is only partly true. Yes, you do pay the same subscription - but most likely not for nearly as long as someone in a raiding guild. There are several ways to encourage people to keep paying their subscription, but encouraging people to team up with and get to know other people in the game is probably the most efficient. Admittedly there are several ways to do this, but giving relatively large advantages to people who are in guilds and cooperate with others regularly is one of the easiest to implement. One has to be aware, though, that many people resent being forced to team up with others. Thus being in a guild should offer benefits while not rendering other ways of playing the game undesirable or impossible.
Another thing I've heard is "DKP is unfair to casuals, it is just a way for the hardcore players to give themselves benefits. Random rolls are more fair." People who say this have never done a large raid with /random loot distribution. I have. My guild took a bunch of randoms (people not in our guild) on a MC run just for fun. Everybody got to /random on items they needed, and they did. Now, what do you honestly think will happen when one mage win all 4 mage drops and one shaman wins 3 out of 4 drops he could use? If you can't figure it out: you'll suddenly find that you have a lot less mages, shamans, warlocks, and hunters in your raid - or in your guild had it been a guild run.
As unfair as DKP might seem, it rewards activity - and if there is one thing that is important in a raiding guild it is to have a core of active players. Without these the others most likely wouldn't be able to do anything. So basically you reward people that are crucial in keeping the guild alive. Thus the guild, just like Blizzard, reward people who are essential to keep the game alive. DKP also adds another very important factor - it forces people to consider what items they want to get. You don't want to spend your DKP on a "cool" item when you know that the result is that you have no chance getting another item you really need that might drop later. This helps spread the loot out among the people in the raid, something that is important to keep people motivated.
In conclusion:Epics are not a reward for paying your subscription. If they were - why wouldn't Blizzard just send them to you in the mail? Being able to log in to the game is the reward for paying your subscription. Just because content exist in the game doesn't mean that everybody should be able to experience it regardless of their situation and play-style. There is no practical difference between nerfing a 40-man encounter so a 5-man team can do it, and nerfing a level 60 mob so it can be killed by a level 10 player.
It is not a reward for logging hours in game, even though this is required (it is an indirect, not a direct, reward of playing a lot). If you spend all your time playing new characters you can't expect to be able to collect as many good items as those who spend most of their time playing only one character.
It is partly risk vs. reward. With the exception of the most trivial encounters, there is still some learning required to master it. Getting 40 people to work together in an optimal fashion isn't as easy as some people seem to think. Experienced guilds do the entire Molten Core in less than 2 hours. Guilds that have never been there before need several days - and the encounters in MC are easy. As I see it: MC is a training instance, you learn the basics of working together with 39 other people. Once the cooperation is working, the encounters become trivial (with the exception of Ragnaros, where you also need some FR gear).
It is partly a return of investment. Learning new end-game encounters can cost several hundred gold in potions and repair costs for each person involved (at least for the first few guilds). If you got the same rewards for doing something that were relatively easy and cheap in comparison - why would anyone try to do it the hard way?
It is a carrot for players to aim for. Thus it loses some of its purpose if it is too easily obtainable. Most people spend a lot of time obtaining items that improve their character. It is one of the core elements of the game. Once your character is fully equipped there is usually little left to accomplish.
And last but not least - it is a reward for activities that Blizzard consider beneficial for the game. They want to profit from WoW for as long as possible, and construct the game in the way they consider optimal for this purpose. Giving people gameplay reasons to join guilds and benefit from being in guilds is one way of increasing the duration they stay in the game.