I wouldn't say it's prohibited by Visa/MC...that appears to suggest they can do something about it.

They certainly aren't able to anything about the Bank of Chinas and Bankcomms of this world.

All we can say is that they are non-compliant with Visa merchant and acquirer agreements and/or VIOR. But there is no consequence. Or even Visa/MC don't want to cut them off (at least acquire the merchant interchange part of the income).