Why do most of the "TEX-MEX" restaurants in the U.S say outside of their locations "Authentic mexican food"?

My dollar goes to Moe's Southwest Grill. The food's great, the service is great and the price is comparable to a quick service restaurant. You get to choose your "fillings" and "toppings". The ambience of their restaurants is warm and inviting. I particularly get a kick out of their crew's "WELCOME TO MOE'S" echo each time somebody walks into their door. I only wish they would do something to their menu names!!