Was he though? We never see any other Bajorans in Starfleet wearing their earring. If anything, she seems to have gotten special treatment in being allowed to wear hers later.
I believe that the regulations probably strictly mean that she can't wear it on duty, but that starship command staff have flexibilty to allow it due to cultural requirements. Picard and Riker both seem to have been completely unaware of the cultural significance of the earring, and assumed at first that it had been simple jewelry.
Riker was being a hard-ass, due to Ro's reputation and the fact she just came of the stockade.
But Riker was NOT a jerk particularly, as this would probably be the normal reaction to anyone taking someone of Ro's reputation on board,