Almost every one I've signed went further to specify that the merchant, so long as they display the Visa/MasterCard/Discover/American Express logo, is specifically forbidden from giving a discount for cash in the transaction.
Naturally, why would they want to undercut their own system by permitting such transactions to occur? Is it enforceable? Likely not, but likely more than acceptable cause to terminate the contract for their accepting cards.
Oddly, Wells Fargo permits a cash discount, but specifically forbids charging more than the marked price for items paid for with the card.
Enforceable is probably a matter of how much you can intimidate a merchant. It used to be fairly common for retailers to post signs at the register with warnings that they would not accept credit cards for any purchase under $X (usually five or ten bucks.) I actually stood in line behind a guy at a retail clothing store who called the 800 number for VISA because the vendor would not budge on that policy for a t-shirt that was like, seven bucks. Minimum purchase restrictions were also a violation of the merchant agreement. Now I see kids hand credit cards over the counter at McDonald's for a milkshake.