Planning to be arrested is a common protest activity. A lot of people don't do it because they can't afford to be arrested. Assuming it's organized and planned and not because the arrestee was being violent or something, it's generally considered positive by the protesters. If the people protesting in Ferguson don't think he's an ass for doing so, why do you have to?
Him being there and getting arrested helps them get those headlines you want them to have.
The headline "dude no one outside of protest circles has heard of get arrested" doesn't help them, especially when the article talks about how he went there to get arrested. It will just as likely drive people away from their cause as help it.
For a similar reason, if I were among the protesters I would ask Al Sharpton to shut up and go away, since he is a more divisive figure than anything else.
Just because you don't pay attention to/don't like black people, doesn't mean they aren't important.