Maybe, and I can't speak for the OP, but my response would be that I don't really expect that many spammers would be reading the denial emails. Also, I'm guessing a lot of forum admins would rather help a legit user get the registration right than to turn them away on the chance that they might be a spammer.
Apologies for my misunderstanding - I thought this was just the vBulletin message given after registration fails, not emails.