Um, no they don't.  If VISA or MC catch a dispensary using their systems they fire the company that set up the account.  What many dispensaries are now doing is using a "square" reader attached to an I-phone.  But they do not have the blessing of the credit card companies.

Until the US legislature changes the law it will remain illegal for dispensaries to transact as every time they do they are laundering money.  (FWIW: PotCoin does not fix this problem, drug dealers using it as a substitute for money are still laundering).