Didn't read Jude's link; didn't watch the full video. I watched long enough to notice a few things (I stopped it at 3:51, so if you want to see what I saw, watch it up until there).
* There is no way to tell if he's dealing with actual police officers or simple security guards. Since most security guards do not have the authority of arrest (at least in MA), it's reasonable to say he was probably dealing with actual police officers.
* It is illegal to videotape police officers in some places. It is also illegal to videotape people without their express permission in most places. And in case there was doubt about the express permission part, the cameraman is asked on camera to turn the camera off. S/he fails to respect that.
* There is an adorable little cut just before the arrest itself, after a lot of ranting. There is no way to know if, between cuts, the signholder yelled a threat at the officers, brandished a weapon, did a sudden jig, or began to levitate. There is nothing showing us what led up to the arrest. For all we know, he told someone their mother's a whore and he's going to kill them. That would be arrestable.
* This is addressed by Jude's post, but there's no indication of whether or not it's actually on public land or not. Most fairgrounds are in fact private property, in which case, yes, you MUST leave if they ask you to. Otherwise, you're trespassing and that is a crime.
So it appears that the man was not, in fact, arrested for protesting Obama, but for actually breaking the law. Truly shady, that. Bad, bad policemen.