Because if there's more spam, it means more signatures, and that means it appears almost everywhere==bigger reach, and who would care about the content of the text so long as the name gets spread all across the internet?
BCT could be monetized much further, but it has not. theymos didn't exactly have to roll out the merit system but he did. Users would still stay on the platform even if you saw banner ads because guess what: the altcoin sections are already so brutally filled with nothingness that it wouldn't change a thing. They'll just run in, post their garbage and then leave. For the majority of forum members, as long as there's money to be had, they will take it. Why do you think those ad programme sites are so popular (PTC)? There's an incentive involved and users don't care. Same thing with faucets, we should all know how ad-infested and seizure laden they are.
For sure, the forum could use various means and methods when it comes monetization, but since theymos doesn't want money the forum probably won't be utilized to its full monetary potential, which is good and bad in their own ways. When money is involved, more people are eventually involved, and the originality of discussion reduces significantly as the clock ticks, and the place intended for discussion turns into a virtual brothel full of hookers and drunk cunts. Sounds about right, innit?
As for your say of banning signatures, stop altcoin campaigns instead of stopping all of the campaigns. I mean, most of the spam is because of fucking altcoins and horrendous shits. Keep only the bitcoin paying signature campaigns and see the difference. Make signature campaigns be available only for Full-members and above.