Partially, I think "they presumably like it" is an invalid assumption, as evidenced by the many youtube videos with more downvotes than upvotes. More important

Partially, I think "they presumably like it" is an invalid assumption, as evidenced by the many youtube videos with more downvotes than upvotes. More importantly, the argument made by campaigners looking to stop ads on objectionable content is that placing ads in association with objectionable content constitutes funding that content; whether you do that via an algorithm or manually makes no difference, the point is that the advertisers choice of how to place ads is paying out money to producers of objectionable content. That, in turn, makes those advertisers liable to explain to their regular customers, who are now vicariously funding objectionable content by paying money to the advertiser, why they are placing ads as they are. All-in-all, it'd be much easier for a generic advertiser to just avoid objectionable content; but that isn't properly possible on Youtube, which is what I suppose is what this whole scheme is trying to change.