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.
Does it even matter what the quality of the traffic is for earnings purposes? I can't say I understand that aspect of how the forum earns money, but I'd think that traffic is traffic.
Quality doesn't matter if you're only considering the metric of forum views. However, the fact remains that these ads are more often than not going to be ignored by the masses of bounty hunters and other sig spammers. Since they don't read anything anyway, why would they bother with an ad? Certainly, the retention of attention is mitigated by the fact that spammers just want to see the title, post, and get out. Genuine traffic from people who care about discussion is more likely to attract users to a platform. They are also likely to have at least 1 bitcoin, which I doubt can be found in the masses of signature campaign members. Certainly, if theymos wanted to, he could increase the ad slot count or even make ads more common. Rather than 10%, make it 50%. Bump up the auction price. But since that hasn't happened I don't believe he thinks that any type of traffic, whether it come from spammers or genuine users, is fungible.