I heard the interview with the owner of the pizza chain on NPR. He made a business decision. A lot of his customers are Mexican and usually have a few pesos left over from their visits to Mexico. He believes that by accepting pesos, he will encourage more Mexican people living in the US and having access to his store, to buy pizza from his stores and use up those peso rather than just holding on to them.
As mentioned previously, many stores located on the border, both North and South, accepts the neighbor's currency. It is a business decision.