Of course, it's perfectly reasonable that if you know someone stole something, you can stop them. Under the prerequisite conditions section, it is stated that:
The shopkeeper has reasonable grounds to suspect the particular person detained is shoplifting.
Wouldn't that mean that someone who has done nothing suspicious other than refusing the check would not be giving anyone reasonable grounds to stop them? Or does just refusing count as reasonable grounds and make the check effectively legally mandatory?
It doesn't sound like they typically check for most items, just the expensive ones and the date on the receipt. That makes it even more theater and less practical.