It used to not reflect on the advertiser, but ever since advertisers started demanding more control over where their ads were displayed, the expectation is now that they all do it. So allowing your ads to be displayed in those places is now interpreted as an endorsement of that content.
Don’t the ads just run automatically on content? It’s not like they’re hand picking these as targeted ads for people who enjoy neo-nazi propaganda.
It used to not reflect on the advertiser, but ever since advertisers started demanding more control over where their ads were displayed, the expectation is now that they all do it. So allowing your ads to be displayed in those places is now interpreted as an endorsement of that content.
Most ads are targeted these days. You tell which user profiles to run ads to.
Also appearing next to controversial content is brand damage for these companies. They try to avoid it as much as possible.