Also, what happens in practice if they refuse. Years ago in a different major US city, the local authorities had set up semi-random TSA style checkpoints for subway stations as a pilot project. When I was on my way home from work they were set up at the station where I'd get on the subway. I refused and left, deciding I'd rather walk home.

I got about 5 mins down the road when I realized a couple of large men were running after me. They were clearly cops, though plain clothes. As they caught up to me one pushed me so I'd stop and they blocked my path. Gave me all kinds of grief, wanting to know why I refused. There was an obvious implication that I'd done something wrong. I gave a few cursory answers to their questions, then started responding with "Am I being detained". The latter only incited them further.

Thankfully at some point they gave up instead of actually detaining me. They chewed me out, told me they were going to radio their buddies at upcoming stations so I wouldn't try to get on there either, and stormed off.