These are known as high risk payment processors. They do this because of the outsized risk of chargebacks that occur against payments for porn.

Wife catches a weird pornhub charge on your monthly statement? Whoops! Those hackers are at it again. Call it fraud and charge it off.

The high cc processing rate accommodates the increased risk.

Btc solves this as no chargeback is possible.

There are plenty of other industries which are just as high or higher chargeback risks and aren't forced through hoops. Gyms are a prime example. Food delivery has a high chargeback risk. Gift cards are a go-to for leveraging stolen debit/credit cards. You don't see the industry forcing regulations here, or forcing these industries to go through specific payment processors.

It also doesn't explain why porn actors routinely find their checking accounts closed out on them, or they get blacklisted entirely.

It has nothing to do with risk, and everything to do with Christian fundamentalists in the banking industry exploiting their positions in industries we need, to force their morals on others.

the problem with free market fundementalists is they deny that stereotypes, emotion and plain stupid decision making are often having greater effect than the hand and foot of the market

This just means the door is opened for a competitor that can provide the same or superior service at a lower cost. Free markets can have efficiencies, but they should be arbitraged away in the long run.