Newspapers, magazines and broadcast news have been actively, negligently and often knowingly defaming people and organizations for decades (centuries?), sometimes with very serious consequences for the offended parties. I've not heard anyone calling for Visa/MC to cut them off.

If there is a legal case to be made against Pornhub, then the authorities should make it. Otherwise they would do better to go after those who made (or uploaded) the offending material in the first place.