This is the way I see it:
Visa has a big incentive to make sure people have confidence when they buy things online. The more confidence they have, the more they'll buy, the more they buy, the more $$$ the whole Visa system makes.
If cardholders go to a porn site and whip out the credit card because they see the Visa logo and they have confidence in Visa, thats all wonderful for whoevere owns that site because they just made a sale, BUT if that site fucks around that cardholder and does sneaky cross sells or re-billing tricks, then that cardholders confidence in buying things online goes in the toilet. No confidence = no future buying online, whether it's porn or some jeans at
www.gap.com , that persons confidence has been shattered by one single site, and will take ages until they might buy something online again for fear that they might get fucked again. Not only that, but they might be having a conversation with other people about buying things online and they'll spread the word that buying online is an exercise in torment and frustration and scams galore. Yay!
So while Visa might still make money from that one sale where the cardholder got fucked, they lose money from all the future potential purchases the cardholder might have made because now the cardholder has no confidence in buying online. Boooo!