> Why do people so strongly critique Facebook for selling ads but not businesses from buying them?

I guess for the same reason people tend to criticise drug barons more than street dealers, and dealers more than addicts.

Though everyone plays their part in an ugly business, there are power relationships in operation. Facebook know what they are selling and are gatekeepers to a near monopoly product. Businesses are desperately in need and find few alternatives these days. Facebook are apex predators and simply carry more responsibility and so deserve more scrutiny for their role.