Spammers are easy to deal with, the infrastructure allows the banning and deletion of posts from a member with the click of a button (and a few confirmation pages to stop you from making a mistake!). All in all it takes a few minutes to deal with a spammer. A lot of other measures take up a considerable amount of time.
I've been giving it some thought and as annoying as it is having them spam garbage, it is the most economical way besides having a 1 hour grace period between sign up and post or an extra confirmation step (automated) that runs via the email system. Good chance is that neither of these are natively supported with this forum otherwise they'd have probably already been implemented
