Because clearly, no one could corroborate his or her immigration status through any other means, like maybe an address that matches up to the name they give, or a social security number, or hell, maybe he gets taken down to county and the great mystery of who the fuck is this guy, and where is he from gets solved there. SO FUCKING BE IT, because if he's here legally then he gets set free, and if he's not, then his ass gets mailed back wherever he came from, postage due.
The problem with this law, as everyone against it would have you believe, is that it requires common sense on the part of the people enforcing it. Since it's such a foreign entity to our elected representatives, they can't fathom how us lesser people could possibly possess it. You see, our law enforcement officers would have to be trusted to use a modicum of common sense and good judgment. However would all the police officers in these hispanic communities, who grew up in hispanic neighborhoods speaking spanish, ever communicate with someone who didn't have their identification on them?! Impossible to overcome, throw the baby out with the bathwater, and shame on us for thinking America was meant for Americans.
You know what Spekkio, you're right, we should probably just throw open the borders, rename ourselves the People's Republic of North Mexico and go with it. You've poked a hole in the entire premise that no one in all the collective wisdom of Arizona could have possibly foreseen. Thank goodness you were here, that immigration crisis thing could have gotten dicey!