When I worked retail back in college, after the store closed a manager would have to unlock the door to let you out of the store. Letting employees out was never a high priority, so you often had to wait a while.
Multiply that times 10-20 employees a night at ~1,000 stores and you've got a good chunk of change.
We had a similar thing with security checks, or 'last person in the store', or whatever.
Basically, people are deciding that they are willing to work for sub-minimum wage, and so they accept it.
Ultimately I just left.
I think the way to fix this is to give employees negotiating power. We shouldn't be in a situation where an employer only treats employees well because of fear of reprisal.
It's one of those things that sounds ridiculous when phrased one way but totally legitimate when phrased another for the same objective facts.
Ridiculous but true: "I'm locked in at work after I clock out and need to wait to be let out the front door."
Legitimate but true: "The front doors are locked after closing and the manager is finishing a task before unlocking it for me to go home."
Presumably there are still fire exits in the building so it's not necessarily a safety issue (or would be more of a safety issue if the front door was left unlocked while the employees clean up).
The problem stems not from being locked in the building, but in not being paid (usually overtime) for that time where your actions are dictated by your employer.
The employees likely wouldn't mind staying 3 hours after work while the manager does Whatever if they were getting paid overtime.
Assuming that there are appropriate emergency exits, and that you aren't delayed after you are clocked out, it's probably not. Fail the first assumption, it's a safety violation, fail the second and, at least in many jurisdictions, and it's a wage violation.
Multiply that times 10-20 employees a night at ~1,000 stores and you've got a good chunk of change.