Because it encourages trolls to create more accounts just to troll the site which creates more work for Super Moderators and Admins, so it's best to just ignore them and let the Admins do their thing.
Banning is pretty stupid anyways IMO. Because if you can ban people at will it's highly abusable. For instance, someone goes to this board and spams porn links everywhere, and cusses at people... they deserve to get banned.
Now on the other hand, someone who says things other people disagree with, or hate them.. they could go to their buddy-buddy mod friend and say "Ban them because I hate them!"