On a more serious note: The rules say no spam. At all. Which basically means that you would risk being banned in a forum for spam only, because the spam in there would be against the rules. It is easier to maintain a strict policy against spam all together, than make exceptions. People are prone to project the exceptions to other sections, where they do no exist, and then complain that they are allowed to do so and so in that section, but not in the other, and that is unfair. Worst case scenario would be that the forums are flooded with spam, because people believe it is okay, since everyone else does it. Less bad, but still bad would be that the regular users would run amok due to the spam and the incompetence of the mods, whether or not the spam was only located one place.
On another related note: You are not allowed to spam in the forum games either, but the mods are often more lenient, because it is games.
