Any firm is free to define what is acceptable as payment for their goods - wampum, seashells, large round boulders, you name it. "Legal Tender" just means that US currency is OK to use for the settlement of a debt, public or private.
Actually an earlier poster was right on the mark- these guys are killing 3 birds- creating a way for people to get rid of (at least temporarily) unwanted currency, making a nice profit on the exchange rate (they're giving 12 pesos to the dollar, higher than the going rate), and advertising as pro-Hispanic.