I'm solidly in the "their house, their rules" camp when it comes to showing a receipt.
They are not the government and (ignoring what I consider to be improper and unconstitutional "anti-discrimination" laws), our interactions with each other are mutually voluntary and beneficial. I have zero use for the kind of scum who engage in theft and so have no heartburn doing a bit to help others avoid being victims of theft.
I know that a lot--maybe even most--theft is from employees, not customers. But I know theft from customers isn't zero so I'll not deliberately make life hard on those who are doing what little can be done these days to discourage and prevent theft out the front door.
If I see someone prowling around my neighbor's house, I will call it in and otherwise attempt to assist my neighbor. I view a retail business no differently.
No, I wouldn't consent to a strip search, nor of emptying my pockets. There are lines.
But if a store employee wants to see my receipt on my way out, I've got no heartburn at all with that. Others' lines are in different places.
I notice that some stores are constructed so that you are more or less forced to walk through a register line to exit. I actually appreciate the more open design of Walmarts and other stores where I can wander around and then exit the same doors I came in, without having to squeeze past a register.
Anyway, my opinion. Worth what you paid for it.