Hmm, I think we are talking past each other. It isn't allowed without notice to MC/Visa, the cap is ~4% of the total, IIRC (which is usually far less than what I remember seeing when I last reported it a few years ago), and they require advance notice to the customer (before consumption) along with a line-item on the receipt.

Just because it is legal, doesn't mean they are doing it right -- and in my limited experience, they usually aren't.