How to Remove Your Head from Your Nether Regions:

 

People who haven't read the law and don't know anything about requirements that resident aliens already have to fulfill.
If you're a resident alien, you always have to have your 'papers'. It's federal law that you carry them.
Under this law, illegal immigration status is not a primary offense - they have to stop you for something else first. If you produce an AZ driver's license (or license from another state or some other official identification document), then you're presumed to be a citizen. If you produce the papers that the federal government makes you carry around, then you're presumed to be a legal resident alien. End of story - you get your speeding ticket and go on your way. Of course, if you're driving without a license, and without papers, you're in trouble. But then, you'd be in trouble anyway for driving without a license.
Link to the law, full text: http://www.azleg.gov/legtext/49leg/2r/bills/sb1070s.pdf