Because they both look "white". If someone saw them walking down the street and didn't know who they were, they wouldn't think they are Hispanic. Put George Lopez on the same street, and at least one person is going to call him a "wetback".
You specifally said "hispanic whites" and I listed two examples of hispanic whites, people call them white because they're WHITE.
Sammy Sosa is black and hispanic, people seeing him would see a black man.
George Lopez is Indian Hispanic and not in the white hispanic catagory that you mentioned.
Hispanic is an ethnicity, not a race.
Again white hispanics are already treated like white people because that is what they are.