I don't think it's really fear of ass-whoopin'. I actually think it has more to do with the fact that if there's nobody around to respond immediately, it's easier to say.
Honestly, I'm affected by that too. It's much easier for me to send an e-mail when I'm steamed about something at school or something than bring it up in person.
It's all about the inhibitions, though, and because everyone on the Internet is (or at least feels
) totally anonymous, the inhibitions are sorely lacking.