Yep, it's an anti-spam precaution. Unfortunately there are a lot of human spammers around these days and the usual spambot-blocking techniques don't work on humans. However IMO this has been worth it, because we were just starting to get hit with human spammers and haven't had any trouble since I put this little trick into operation. If they can't post links they will piss off and leave us alone.
Obviously we are not going to publicly state the required postcount before this restriction is removed, and equally obviously we will change the required postcount if that becomes necessary. Also, any spammer that does manage to sneak past will be banned instantly and have all their posts removed as soon as they are discovered.
Devious, treacherous, murderous, neanderthal, sub-human of the West.