The fairest I can be there is that it might make their ad sales more complicated. It'd be more difficult to count "impressions" for an advertiser if you had to

The fairest I can be there is that it might make their ad sales more complicated. It'd be more difficult to count "impressions" for an advertiser if you had to work out whether an ad served through the API was actually going to be shown to a user. The advertisers might also dislike the lack of control over the context their ads would be shown in. This isn't insurmountable, but it does present some challenges.