Because he is actively causing friction. He is striking against something that obviously distresses them, and quite simply doesn't care. Expecting the whole world to be jehovah's witnesses that ignore all rudeness and hatefulness from other people isn't really very realistic.

If someone went around poking someone in the ribs with a stick, we wouldn't berate the pokee for not tolerating it. But if it's verbal or mental poking, then suddenly the person on the recieving end is the one at fault for objecting. I wonder.

EDIT: By the way, I DO feel differently about "friends" or "constant aquaintances" like coworkers. That is what I am talking about, not just general people you'll pass by or encounter only at rare times. No, it's not reasonable to expect that you know what will offend people you don't know. But to deliberately attack friends with something you know bothers them because you think it's funny is wrong. That's my opinion. I don't do it. I don't stay around people who do it for very long. That's not a good friend, in my opinion.