Ads are placed in ways that don't directly connect to a sale already. Billboards, TV (last decade), youtube sponsors, even at least half of traditional internet ads. Generally they'll just see whether sales increase after they start running the ad campaign, if so, it works.
In every single case you've mentioned you're going to have a campaign with an attempt at attribution. Certainly youtube sponsors all use direct attribution with "use my code" etc.
In every one of those cases you can at least do things like count viewers. How would that work for a model that's just subtly biased?
That's missing the point, I think. With those, you (as an advertiser) can verify the ad yourself. With an LLM, especially a subtle one, you can't.
Just ask it the best brands of X