We state it as true, by observing spam variances since paid advertising's induction to now. It is absolutely conclusive that paid signature advertising has increased spam to an astronomical level. People that do paid signature advertising as the only way to earn Bitcoins, rather than spend 20 hours per day forumlating hundreds of thoughtful posts, spend 30 minutes posting hundreds of times per day, giving the moderation staff hundreds of posts per day to delete and users to ban.
That you list my comment in bold, does not make it any less true. The prior statement remains false on its face simply because it expresses an erroneous conclusion, most simply because advertising in signatures has both positive and negative aspects for the community. It is not universally a negative, for reasons previously stated.
As far as what moderators do? That is what moderators do. As a moderator myself (not here), it truly takes very little time to moderate people, and if I was ever unsure (and about spammers, I never was) I would just ask someone else's opinion as well.
If you think the sponsor has time to look through tens if not hundreds of thousands of posts per month to judge if they are spam or not, you would be incorrect. They can say they only pay for substantial posts, but they dont have the staff nor time to read through everyone's posts.
You cannot possibly hope to speak for any sponsor, you have no idea what time they do or do not have, let alone how much time they chose to spend reviewing their employees. By extension, they have every incentive to insure that their paid sig users are doing their best for the community. Similarly, they have zero incentive to pay people for posting idiocy. Much like moderating, it just doesn't take that long to scan through someone's post log, especially if they wish to better their business.
None of this, of course, is meant to downplay the negatives, but to suggest that some heavy-handed statement that all ad sigs are bad for the community is mindless hyperbole.