@Len - I've come across many with open comments and no admin at all. One got hit by a Reddit / 4chan battle a while ago - an old post - and the battle raged for a whole day before I sent the owners a quiet note and they removed all the offending posts. Comments are, however, still open to all.

I can understand why some simply remove the link: more comments makes a site seem popular. I've noticed that spam commenters don't register as a hit with my visits on Google Analytics and so on, perhaps that makes a difference too?