Some people only seem to get their pleasures from making other people miserable. Before the Internet, only the most pathological of these could make a living because the locals would always get to know them and stop putting up with them. Nowadays there's always a new site or a new anonymous ID to use. (Did they always exist but just learned to live without before the Internet, or did the Internet make them? That's ultimately a meaningless distinction.)
In most Internet communities these people are called trolls and it is universally agreed they are a problem (except by the trolls themselves, of course). However, in some competitive games, such as MUDs and later MMORPGs, they are called griefers, and their role is less clear. Particularly in those where people pay for power.
Consider particularly the case of one of the Iron Realms games, the biggest MUDs in the world, like Achaea (which I never played) or Lusternia (which I do play). In these games, it's free to play, but you can spend cash to buy boosts in skills or special artifacts. Certainly, there are reasons for anyone to buy these boosts, and some have nothing to do with fighting. Nevertheless, the average griefer tends to spend more than the average for the general population, quite a bit more. (This is also true, perhaps to a lesser extent, in games like WoW where the pay-for-boosts thing is more marginalized and less over-the-counter but still present.)
This puts the administration in a tricky balance that they might not even realize they're in. Let's change metaphors and call these people wolves. They're never happy unless they have victims, let's call them sheep, to make miserable; and they generally won't let up until they have to, often because the sheep flee. This is no problem for the wolves. There are always more sheep, and if there aren't any on one particular game, there's always other games full of more sheep. So there's no reason for them to hold back. Sheep are a self-renewing resource, as far as they know, so why not indulge their instincts?
But if you run one of these games, you don't want to drive off the wolves since they're worth good income, better than anyone else maybe. But you also need the sheep. First of all, sheep also produce income, not as much, but they do. Second, if there aren't sheep, the wolves will get bored and leave. So you need to encourage the sheep to come and stay -- even encourage them to buy boosts because that'll be income and it might also make them stay through more harassment -- but you can't go so far as to protect them from the wolves beyond a point. You need the sheep to be happy enough to stay long enough that it will keep the wolves happy enough to stay too.
Thus, your game's fiscal success depends on keeping everyone just a little unhappy, but not unhappy enough to leave too soon. Alternately, you could run a wolf-free game, but not only are the wolves your biggest spenders, even the sheep tend to get bored in a world completely devoid of wolves. What the sheep want is wolves that add a bit of excitement and then leave off; but almost anyone who'll fill the wolf role will also do it too much. So for a sheep (and note that I'm one) the best balance is a world that has wolves, and takes some steps to contain them. But the sheep's ideal world always has somewhat more containing measures than the game actually has, because if there were enough to make the sheep really happy, the wolves would leave for greener pastures.
I wonder how many of the people who run the games realize this consciously. They certaintly wouldn't admit it. It would sound crass and mercenary and manipulative. But you can't run a business without thinking this way, and acting on the thoughts.
(Incidentally, I changed metaphor not to give them a better tone, since wolves are a valuable and respect-worthy part of the ecosystem, but simply because I needed a word for their victims that worked well. But though griefers do add something to the "ecosystem" of a game, it's nowhere near as valued or needed as what wolves add to their ecosystems. So don't be misled by the metaphor.)