Arbitrary rules like that are a way to teach children impulse control, restraint, and patience. I can't easily find the exact passage, but The Better Angels of

Arbitrary rules like that are a way to teach children impulse control, restraint, and patience. I can't easily find the exact passage, but The Better Angels of Our Natue talks about how it's improper in English culture to use a knife to help eat peas. He credits these kind of arbitrary rules for helping reduce violent impulses. (Note: I read the book a while back so I'm probably misrepresenting the author's point in some way, but I think that's the general idea.)