In my experience, the rules are not that strict, unless there's obvious advertisement involved. When someone asks for an honest opinion about some site, the thread usually doesn't get deleted.
When, however, someone starts a thread about "which site logo do you like the best, and oh, by the way, on a completely different side note, I'm the owner of this site, check it out," then it's simply spam.
I personally disagree with this rule. You may also, or may not, but I will explain myself on how I disagree with it. We all know the drill- in a "our site vs their site- who's better?" thread, everyone says that the site they are posting in is better, and I think a lot of them are because they don't want to get in trouble with the owners of the site. Usually the answer in these debates are "both sites are ggod, and both are worth joining", but people again don't want to say that, for the same reasons. This rule was presumably made to prevent spam in threads which have nothing to do with other chess sites. Because of this, I see no reason to delete posts such as "I think their site is better or equal to us (in a thread of that topic) because...", or, "Hi, my handle is suchandsuch in suchandsuch.com, just to let you know (again on an appropriate thread)". I think it should be allowed to have a dual promotion. Members of chess.com (honestly) say what they think about another site they are in and spreads the word, and members of that site do the same about this site. The mebers benefit by getting introduced a great site, and the owners of both sites benefit by getting new members. Just think about it.