Privacy implications aside, the LEGAL implications actually delight the dark parts of my mind.
This is blatant over-reaching. If ANY prosecution is attempted based upon one of these vans acting as a primary cause of law enforcement action, they'll be off the streets in a new york minute.
There's no probable cause, there's no reasonable suspicion. There's no commercial transaction to hide behind to call such an invasion of privacy a "voluntary screening." After all, if I don't want to be backscattered, I can fly myself or I can avoid commercial air travel. There's no way to avoid a van randomly scanning anyone/everyone around it on city streets.
I also wonder how this works considering particular wiretap laws. It's no different than a small spycam underneath a person's clothes or (if used through a building) inside his/her home.
Count me in with the guys with tar, feathers, pitchforks, torches and rifles, if a judge actually approves of this when the inevitable case comes up.