I think there's two factors at work here. Number one, the ad buyers have a brand image they wish to control, and therefore they have an interest in ensuring that their brand is not associated with content they think would tarnish their image. Even if every ad-viewer consciously knew that the ads did not imply endorsement of the content, there are subconscious factors to consider (factors which form the basic premise of a huge percentage of corporate advertising--eg that positive subconscious associations with brand imagery is one of the purposes of advertising).

Second, this is a (needed) power play on the part of ad buyers. Google has growing control over huge swaths of online and mobile advertising. Ad buyers need to push back, questioning the value of this sort of advertising and demanding more control over where their ads are displayed. Why should they trust Google to do the job of deciding where their ads belong for them? Google has proven time and again that they trust their own AI and machine learning tools far more than they should. And the longer they wait, the worse it's going to get.