I suspect it wasn't even as much that it was an anti-Obama sign but rather one advertising a private group and directing traffic to a website. Doing such things on private property, where others have paid for the right for such advertising, will often result in being asked to leave.
For how he was behaving, he was pretty much being handled with kid gloves. Now if he had been tackled to the ground (instead of going down after wrapping his legs and clinging to the security guard's leg), and placed in a choke hold UFC / Forever21 style, maybe you could say he was "brutally assaulted" by security.
Had he talked to the head of security at the onset, instead of shouting back at the top of his lungs, this probably would have been a total non-issue. Instead the guy got the attention he wanted and will probably get the fine he deserves.