I've also noticed that when you (generalized 'you') discipline your children in a public setting, other parents tend to follow suit. It's almost like some parents are so afraid of being tagged a "helicopter" parent that they let their kids run wild, but then when they see that other parents are chewing their kids out they're less shy to do the same.
I also agree 100% about thanking the parties who show manners. Last year I encountered such a polite young girl (probably 8/9) at a concession stand and I wanted to tell her parents how polite she was, but they were too far from me.