These aren't "Constitution free" as much as "Constitution ignorant". Go to youtube and see the proper way to respond to an unconstitutional stop.
Repeatedly ask "Am I being detained?" and "Am I free to go?" and mark the exact time (with the DHS guy watching) that the stop begins. There are time limits of detention without cause. Be polite and respectful, but your are NOT required to "show your papers" to this idiots, no matter how much they try to intimidate you.
Then, if you are unlawfully detained, get a Constitutional attorney and take that shit to the mattresses.