I don't think you could make the case for less separation of search and ads from this.
Google probably sees forum spam as posing a significant risk to the advertiser. They've been banning people for posing a significant risk to the advertiser for years. (For all I know, they've been banning ads on forums with spam for years too, I just don't keep up on forums cause I don't run any). If there has been a change, it could well have been because of advertiser complaints when they review their placements and find their ads in with the spam.
The bottom line is, you're entirely responsible for the environment(s) in which your AdSense account displays ads. It's up to you to keep it tidy.