One mans "censor" is anothers "brand protection". Given that they are currently allowed and expected to decide who gets to use their payment networks, it seems pretty obvious to me that they won't allow extreme content next to their logos. If you made clear rules that specify when they have to extend service, I'm sure they would love to relinquish that responsibility. Visa has no moral objection to what you do, as long as they make money and aren't exposed to political pressure.