The issue here wasn't that a security person took down the poster, it was rather than call someone higher up and ask "why the hell is a rent a cop touching my personal property?" he decided to go to name calling. That is not how things are done in business if you want to keep your job. You call your boss, he makes a call, a couple hours later either the matter is dropped and you put your poster back up or your boss calls you a douche for putting up a dumb poster. Either way, quick resolution.
But no, let's put up a new poster implying the security people were fascists. Because that will help. Now everyone gets called to the (metaphorical) principle's office because your couldn't handle it like an adult.
Oh and BTW, they took down a poster in your work environment, not beat you for stating political ideals. There's a difference.