You're half right. That's the only thing they're obligated to do, in the positive sense, but you're forgetting about negative obligations. A negative obligation is an obligation not to do something. For example, while one could argue that you're not obligated to go out and save people who are in trouble, one would argue that you have a negative obligation, that is the obligation not to kill someone.
Musicians, regardless of who they are or how big their fanbases are, operate from a privileged position ie: one of power. In the positive sense, they are merely obligated to write music (not even that really, they can quit whenever they want), however, they have a similar negative obligation, that is, to not abuse their power/position over others. I would contend that's what Max does time and time again. He tends to rally his more fanboyish (or fangirlish) followers around his cause, and then uses this to get them to do a certain thing such as attacking that girl on twitter or yelling about Tim Kisnella's opinion. That is abuse.
You're right, he probably shouldn't be considered a role model, but he is because he's in a band that people like. This would be the same if he was just in a popular local band or if he was in the biggest band in the world. He has an obligation NOT to be a dick ub certain cases because, in being a dick, he can do more damage than someone like David can. So yeah, in one on one interactions, Max can act however he wants, but at the point where the situation becomes public, he has obligations that common folk don't have.
so yeah to sum up, I don't have a problem with him sparring back and forth with David (other than specific dumb things he said, his tendency to call people who disagree with him nazis is annoying), but in the case of the girl on twitter, there's a problem.