That phrase is not what one would consider hate speech or particularly offensive I would think but reading a story on the event from ABC News, a couple visiting the museum apparently told Mr Rodgers something different about his t-shirt.
"One family of religious people told me that I had ruined their trip, and they drove all the way from Virginia," said Rogers.
So they were offended by that were they? I wonder how they would feel if they were made to pledge allegiance to a god they didn't believe in, or maybe swear an oath to said god when in court? How about seeing faux stone tablets with a moral code attributed to some make believe god placed in government buildings all through town and praises to an invisible deity plastered all over their currency?
Somehow I think a t-shirt with "there's probably no God, so get over it." is quite tame in comparison and think that maybe they should really get over it.
UPDATE: I've been told in the comments section that Derek Rodger's t-shirt actually read "There's probably no god, now stop worrying and enjoy your life" which is as equally unoffensive if not even less controversial than the slogan reported by ABC.
Either way here is Derek Rodgers explaining the incident.