They are arguing that it would be unconstitutional if only a certain group, like green people or something, smoked weed. The problem with that analogy is that not all illegals are South American and the law is not set up to single out a certain race of people or whatever, the rules for illegal immigration are applied equally to all people. In fact, illegal racial minorities from Africa and Central/South America are often treated BETTER than illegal whites from Europe or whatever.
What I don't understand is why people just assume someone is racist if they don't like illegal immigration instead of assuming that they just don't like ILLEGAL immigration.