You are free to ask for a price in pesos but realistically, you have a buisness that is primarily geared towards Americans (secondarily to Canadians), so they price in the American currency.
Look up the prices for the all inclusive resorts and they do the same thing. They advertise prices in the local currency.
For most it's nice because it takes away variability. While currently it favors the park owner, if the dollar falls, the owners will typically honor thier rates. In either case given enough time everything will adjust.