It's not just advertisers, it's any sort of controversy whatsoever. Monopoly platforms and walled gardens (FB, Twitter, YouTube etc.) don't want to deal with controversy, so they'd much rather silence such opinions. Federated or decentralized platforms are the only way to get sustainable freedom of expression on the web.
I think that's only half true. If you were following the controversial social media engagement by the Trump campaign, it was clearly established that having two emotionally invested warring sides on either side of an opinion is a huge driver for platform engagement. So I think to some extent these platforms need highly contentious trending topics to be "relevant."
