I said why in the sentence before the one you ran off with. Because they get higher than usual chargebacks.

This is due to the "I shouldn't have spent money on that" feeling when the user is done with the service. You'll also have fraudulent cards because there's no physical location they need to order things to to test that the card works. Probably an aspect of underaged users using parents cards due to the nature of the content and a cherry on top in the form of money laundering as the services allows the user to receive money.

As it's a service not goods it's much more easy to chargeback. I'd imagine places that sell adult products have less of an issue, because they have proof of goods.