Get caught without your "papers, please!" and you're hosed. Guilty (of being here illegally) until proven innocent. Hmm, let's deport some of these folks... You say you're Italian or Lakotah? A likely story! What, your wallet was stolen by some gang guys in Mesa, Arizona? And you're just visiting? Yeah, right. We have ways of dealing with you!
And now you're off making big rocks into little rocks until they can shuffle you across the border. Nice.