I don't understand the question. Are you asking why "internet friends" can be so insensitive to each other, or whether there's a reason why strangers can be so insensitive to people they've never met? If I know the people I'm writing to, I am much more aware of their feelings and the effect that my words will have on them than if I'm writing in "virtual anonymity" to people I've never met.
I think, in general, when people write messages on their computers, they're not really aware of other people, so there isn't any check on their behavior as there would be if they felt that they were being observed in public. You should also consider the type of people you're dealing with. When you're dealing with people who basically despise other people and ooze negativity, you should always expect rude outbursts. They probably aren't much better in real-life situations.
I think it's also important to consider why people are writing messages. For example, if I'm having a conversation with someone, whether in real-life or on the internet, I'm very aware that I'm actually talking to someone. On the other hand, if I'm just posting messages on a message board that aren't addressed to anyone in particular, I just don't expect to have much effect on anyone. I take what other people say seriously most of the time, but I don't get upset when I read other people's messages, and that's probably why I don't expect other people to get upset by messages. I admit that I'm pretty sarcastic sometimes, and I do have a tendency to throw other people's words back at them to demonstrate the absurdity of their ideas. But it is never intended as a personal attack; I'm simply amused by irony, and I think it's important for everyone to realize how fallible our beliefs can be (even my own). I wish that we all knew the truth about everything, but we have a very long way to go.
Nevertheless, I'm not going to cop out: I know that my words might upset other people sometimes, but they need to get over it.
My messages couldn't possibly be the worst thing that could ever happen to them, and they need to learn how to cope with more important things. Is that harsh? Of course it is; but it's for their own good.
(I hope that people who know me well enough can tell that I was being sarcastic. If I can't indulge in self-referential parody, then what's the point?)
Anway, if you're asking why it's easier for friends to be insensitive to each other over the internet, I have wondered about that, too. For example, lately I've been hearing a lot about how people have been breaking up with each other by using email and instant messenging services (including cell phone messages and so forth). I guess it's a way of avoiding the other person and pretending that there aren't any consequences. "Out of sight, out of mind." But that isn't the same kind of insensitivity as making remarks to people with no regard for the other person's feelings. I mean, when you break up with someone, you expect it to hurt the other person, so you're not really "insensitive"
per se; you just deliberately want to avoid looking the other person in the face.
That's not the same thing as saying anything that comes to mind when you're not looking someone else in the face. I think the explanation for that is twofold. First of all, people are in their own little world when they're by themselves in front of their computers, and second of all, they're probably rude and insensitive to other people sometimes even in real life.