Personally and for what it's worth, I've never seen the practice of changing username being applied.
This is the first website where I've seen it done, but I thought it was upon request of the user...I guess not.
It's his house, his rules, if you don't like it you can always leave. That said, while it's just fair that he doesn't want his cereals being pissed on in his house, changing the username without the user agreement kind of feels like pissing on the user's cereals...
I think the general rule is to ban the users that post inappropriate comments...