I think that’s largely for the same reason; their legal obligations to ensure they don’t facilitate illegal stuff means that the risk of working with companies that do e.g. amateur porn makes the potential consequences (financial processing ban, i.e. effectively the entire company being shut down) massively outweigh the potential benefits.

So you’re right that PH’s legal liability was part of the reasoning, but that pressure largely came from payment processors, for whom the legal consequences are more severe.