Paypal's ToS explicitly forbid the selling of adult content, there's no clause where it says you can appeal or get approval. So I'm not sure where you've got that information.
Steam is enormous and has the power to dictate their terms to payment processors. Also, the process bulk payments to merchants and as such take on the risk of chargebacks and fraud, shielding companies like Paypal somewhat. Patreon is the same, they have an agreement with Paypal. itch.io worked similarly, but apparently they've decided they don't want to have the administrative burden of handling bulk payments for some of the developers on their platform. As for buying porn with a Paypal account, those transactions are handled through high-risk payment processors like Epoch and CCBill who take on the risk of chargebacks and fraud for the credit card companies and charge a hefty premium to their merchants.