I'd argue there are ethical issues in letting them turn it off. It makes the user less aware that the site they're using is accepting money from that ad source.

I'd argue there are ethical issues in letting them turn it off. It makes the user less aware that the site they're using is accepting money from that ad source. It reinforces their bubble. And it overall makes them feel like the customer, when they're really not, which is deceptive. If FB let users click that button, but secretly broke the functionality, then that's a different issue.