Those folks would be good candidates for accusing of shoplifting. Because, oh, they were.
But I shouldn't be a suspect after purchasing something. Accuse me formally, or get out of my way. I'll be polite as you allow, Mr. Door Guard (who has been put in this position at someone elses direction and I don't resent personally), but if you hold me without the formal accusation (and the rights that gives me), then it's a form of kidnapping, and not kosher in the USA.
The US Constitution: It's not just a good idea, it's the law.