Your link doesn't say that though. Where does it supports the claim that suspect-customers were/will-be illegally detained?
Do you have some other source? And I don't mean "it happened once or twice before the company got slammed with a massive lawsuit that made them substantially change the policy."
Not sure what your understanding is, but imo if a person is physically blocked from leaving a store, then they are being detained. It doesn't matter if an employee is doing it or if it's a gate with a receipt scanner
They can and have had employees standing at exits to check receipts in the past, but people are under no obligation to present a receipt in order to be allowed to leave so most people just walk right by
Physically placing a barrier in the way to prevent that is detainment. Replacing the employee with a scanning machine doesn't somehow change that.
It's not just not a strawman, it's already happening, even if it is illegal