Regardless, I think the warning is better than no warning. Again, we don't know the process behind the scenes.

When it's detected on the ad serving side, the action is "google doesn't show that ad". This is not something that a user will generally notice.

Consider the likely interaction: (a) Spammer tries to figure out a twist on the ad that makes it through the inappropriate ad filters. They keep at this until they get an image or wording that works. (b) Slightly different system goes and tries to find malicious sites. It detects a site where the spammer managed (a) successfully, because it uses some different methods of identifying the bad stuff.

I don't find this kind of result surprising at all, particularly given how big Google is. If the site safety team is different from the don't-show-evil-ads team, it's almost an inevitable result, at least, in some point in the evolution of the system(s) and processes involved. It does point out some improvements that are needed.