Mostly because there are people who legitimately deserve it. Also, because forums are generally collections of people with very different opinions and personalities, and this tends to clash a lot. It's a small community, so grudges stick around. Finally, if you notice, we have quite a lot of unwritten rules here. They don't get you banned, but there are some secret standards that I don't even think could be put into words and when users don't get used to them they get stomped on. This might be why a lot of new posts seem to be really idiotic - in other communities, they might not be so bad.
The lack of moderation actually helps remove stupidity. Let's look at the Roblox forums. If someone acts like an idiot but doesn't break any rules, you can pretty much do jack stuff. You can't really call them out without fear of being banned. So people stay stupid and don't have any sense of common sense, because they're still inside the rules. Here, idiots get pulled apart limb from limb. So we either get rid of the idiots, or they eventual learn to not be idiots. So we have less idiots. I'd rather have less idiots than less hostility. Granted, the forums really aren't that hostile if you don't start any stuff and don't say anything stupid.
EDIT: I notice that most of the people that complain about this are ALSO the type that post stupid stuff. Candy Apple isn't actually that bad, but he's not complaining, he's trying to make a legitimate discussion.
