If he used self-checkout, it would be entirely possible for him, or anyone else, to have stolen something.
I have no issue with presenting my receipt, in fact, I offer it to the person at the door as a courtesy, sometimes they look at it, sometimes they don't. If you've ever been a retailer, as I have, you'll know that shoplifting is a serious problem, and you will have people who just saunter out the door with items. There are some people who view it almost as an entitlement.