It's a valid question. If, for whatever reason, someone wanted to purge their identity from the Internet Archive as much as possible--including from properties that they did not control--I suppose they could try, but I doubt they's have much luck. For example, if you were to ask them to expunge any HN page that you had comments on. One issue is that there's no easy way to delete only your content if you're embedded in other discussions.
I expect it's a legal gray area that mostly works in part because most random forum posts are pseudonymous.
