Blame the beer companies.........the American beer companies.
Beer companies didn't invent Cinco de Mayo as an American holiday, just like they didn't invent Kwanzaa (and even if they did it would be beside the point). Wal-Mart, remember, is the store that pulled that barbecue sauce that featured a confederate flag. Wal-Mart has no problem selling Kwanzaa articles (a blatantly racist holiday) because the cultural/political establishment has no problem with Kwanzaa, quite the opposite.