I believe I posted something similar to this as a reply. Its a mix between a lot of things. Mostly that people feel protected behind their screen, and that the other end of it doesn't effect them in anyway and that people often don't care for those whom they've never truly met.
Its a shame people feel this way and the second reasoning of mine doesn't just happen on the computer, but everywhere. Its the foundation of prejudice. Judging someone before you get to know them, and in this case being rude or intolerant to them. Whether conscious of such a judgment or not. Man often judges subconsciously, which is why it can be difficult to rise above it. And when the person has no direct effect on you besides doing something in-game or with words, it makes people feel secure of such judgment and put actions upon it.
Hm. Just a thought