I think that reveals the motive: they don't want the line to be fast.
Isn't the purpose of the screens less about catching bad actors and more about cultivating a culture of fear/suspicion, and hopefully getting a few effective informants out of the thing?
If the lines go to 2 minutes, or I get paid while in them, where's my incentive to rat out a coworker?
If amazon isn't paying for your time in line, they have no incentive to make it fast. They can invest the bare minimum to protect their own interests, and fuck over their worker who have to wait in line. If they had to pay workers for their time then there is an incentive for them to make the line move quickly.
I can't imagine a setup that is more hostile to your fellow humans as forcing them to waste unpaid time.