If it was also the feature users could use to change their displayed username, it would have to work that way.
I don't think so - it's clearly unreasonable to expect every instance of a username to be policed like that, particularly considering cut-and-pasting quotes. I'm fine with the idea that if it's in a thread it's no longer 'yours.'
But I also think it should be possible to remove ones' own account and have the username on those posts no longer point to an active account.
I also think there should be a "post anonymously" option as well, which there practically already is in the form of throwaway accounts. Might as well just make it official.