Rules on confronting shoplifters likely depend on whether the goal is to get the merchandise back or to prosecute.
When I worked at a small boutique we were supposed to try to get the merchandise back. ie: I noticed you just put a necklace in your pocket sir, please allow me to hold it at the register for you while you continue shopping. That gave the thief an out, and most would take it.
When I worked at a large department store it was all about prosecution, so only management or security personnel who had been specially trained could confront shoplifters. Problem was one employee had to witness the shoplifter pick up an item, conceal it, and exit the store without paying, and the shoplifter could never be out of sight during the entire process. Catching people was tough.
I always wonder what happens in some of these cases where employees confront shoplifters while they're still in the store, because you'd never be able to get a conviction doing that, or at least that was the case when I went through training. Of course laws vary by location, so maybe that works some places.