[RememberRonnie]

I worked for UPS in the 1990s and was briefly in the Teamsters’ union. (Yes, I was young and stupid.) When employees report to work at a hub or other sorting facility, they get a cursory bag check and have to walk through a metal detector going in. They also have to have a more thorough bag check and again have to walk through a metal detector on the way out. There was often a line on the exit side.
The time clock was inside the building, well away from the security checkpoint. You did not get paid for the time at the security checkpoint either way. The union did not care and that was never a point of contention for them.

In the case of a "cursory" check that takes a minute or two, nobody is likely to make an issue of it. The reason this became an issue is because of wait times on the order of half an hour. By giving Amazon a half-hour of the employee's time free of charge, this arrangement removes the incentive to make the security system efficient. It's the same dynamic that tends to happen whenever one person can shift the costs of his decisions onto somebody else.

[Spktyr]

During the Christmas rush, it could often easily take half an hour or more to get out through the UPS security checkpoint. Again, we didn’t get paid for that time and the union would not do anything about it.