Possible, but I think it's something even simpler. They're never going to have the manpower to get rid of all of it. If they admit it's an issue and start trying to address it, they've taken responsibility. And taking responsibility is a liability. (Thanks lawyers!)

If they deny deny deny, they can walk into every lawsuit claiming it's shocking to them that whatever happened on their platform. There is a monetary incentive to downplay it instead of doing the moral thing. Hell, if they did the right thing, they'd probably get sued by their shareholders for opening themselves up to the liability.