Here's my guess, based on nothing but my broken logic.
Haters hate, because by hating on something, you're stating to whoever's around you, that you are different from whatever you're hating on.
Either showing that you are something, which is more or less opposite, or at least different from what you hate, or simply that you are in no way like what you're hating.
Basicly, it's stupid in much the same way as people trying to define themselves through what music they're listening to, or what clothes they wear, thinking that listening to a specific kind of music, or wearing a different set of clothing will make them a different person.
Public figures/celebrities are just easy targets for hating, because everyone knows about them, not nececerily because people are jelous about them, though i wouldn't mind their income.
Note that i'm talking about the kind of hating described in the OP, where people go out of their way, to hate on someone or something that doesn't affect their lives.
Hating on publishers for lame DRM is not mindless hating, since they're screwing over your gaming experience
Hating on politicians who are screwing over your country is not mindless hating, cause (you belive that) they are screwing over your country, fucking up peoples lives to some extend, including your own
hating on justin bieber is mindless hating, because he most likely doesn't affect your life/interests in any significant way, and if he does, it probably has more to do with the people playing his music when it annoys you, than him actually making it.
Cracking a joke about a celeb is not hating in this way, it's a cheap laugh. I don't hate justin bieber, i don't really care about him, but i won't shy away from cracking a joke on his behalf, just like i won't be shy to crack a joke on my friends behalf, or my own, and i don't hate them or myself either.
But in the end, haters gonna hate, and everyone is better of just not givign a fuck about them, let them blow off their steam, take it for what it is, and move along.
