It's probably a matter of having to enforce that rule, or else you would open the floodgate on all types of pitches. I have seen this happen on other sites. On one forum I belong to, they have a turnabout Friday policy when sales pitches are allowed, and that works okay because generally, if I'm not looking for something, I know to just delete the Friday email.
That forum (which is not related to quilting, fyi) got very hard nosed about all sorts of policies and the result has been that it has shrunk in membership drastically since I first joined it about 12 years ago. I think that QB is by far the best run website of its type that I've ever seen, and the moderators deserve a whole lot of credit for their sensible policies and close supervision. A ton of work goes into it every day!