I'm surprised they let him go that easily after catching him shoplifting. They should have called security/police/Aurors/whatever, then there would have been an interrogation and a written report. The fact that he is a foreigner shouldn't make a difference. Maybe Umbridge has some sort of diplomatic immunity, but Ron certainly doesn't.

I'm also slightly surprised Ron hasn't been caught shoplifting in Diagon Alley, but maybe Molly is a bit more careful about watching him.