This is just good business. If someone has pesos to spend, and wants to spend em, then let them do so, get some pesos, and exchange them for a buck.
It's a round about way to make a buck, but if that's what ya gotta do to fatten the bottom line, then do it.
It's an easy way to make money because you will need to convert to dollars at some point and bear conversion cost for that. so you can add the conversion rate into the peso price and add on a commission for the service. ususally with higher fees than a conversion by a bank. If the person is to stupid or lazy to convert at a bank then they can pay the higher commission at the taco stand.