Excellent analogy! Incorrectly applied, though. But still useful:
Child molestation was and still is illegal, and convicted child molesters were and still are required to register and carry ID. The Arizona law, if applied to the child molester analogy, gives the police the right to question anyone, at any time, to ascertain whether or not they are a registered child molester. Sounds OK, right? I mean, child molesters have committed an illegal act to begin with, right?
So let's say you're a good parent who has never touched your child's private parts for any reason other than to bathe them. You're certainly no child molester. You could be out at the mall, with your kids, and a policeman could decide that he doesn't like the way you look at your feet when he looks your way (hey, how could he know you're an engineer?), so he comes over to you, in front of your family and perhaps some friends you have met up with, and says "May I see your child molester registration card?"
You say "I don't have a child molester registration card. I'm not a child molester!" Your friends look at each other. Passing mall-goers gather, wondering if they will get to witness the arrest of a notorious child molester. At this point, the policeman is authrorized, by law, to use professional judgment, perhaps based on some training, to decide whether to haul you in to the police station for further investigation, or let you go.
That would be pretty embarassing, wouldn't it? Infuriating? Degrading???
Not to mention seeming a little unconstitutional, what with all that guilty-until-proven-innocent stuff. I thought we were all about protecting the Constitution around here?
That's why people are upset about the Arizona law.
Good analogy!