I think people sign up for it because they have the extremely mistaken impression that being a member confers some sort of social status. Like, look at me, I get to use the special line because I'm just that important.

It's amazing what a tiny carrot will make people give away control over their bodies. Once the government knows your iris pattern, you can never as long as you live regain control of that information. It's not like a credit card number or even a driver's license number or address that you can change. Even a Social Security number you could voluntarily abandon if the situation were critical. Your iris pattern is more permanent than the infamous prisoner numbers tattooed on people's arms. I have grandparents who survived the Holocaust but lived with tattoos. Trusting an enormous faceless government bureaucracy with your indelible bodily identifying marks, even given our currently open and free society, strikes me as incredibly stupid. How do these registrants know what might be done with that information twenty or fifty years from now?