how do all these other adult sites get away with charging premium subscriptions?

Porn sites use smaller special purpose payment processors that have higher fees. It makes sense for porn sites because they only sell sexual content. If Valve and itch used those same payment processors, they'd be paying the higher premiums on all transactions, including those for nonsexual content (i.e. that vast majority of their transactions). It just doesn't make sense for them from a financial perspective.