I think it can be both
In fact, I think if it weren't for the moral aspect, they wouldn't really get away with it. Imagine if they suddenly got a large problem of charge backs for groceries, gas or prescriptions. They might want to stop accepting payments for those things but the backlash would be astronomical against them
I've also never actually seen any concrete numbers about how many more chargebacks they get for adult content versus other categories. Is it possible they really don't get that many more chargebacks, but since it's a category they feel comfortable going after they decide not to accept those payments anymore?