I was at a large branch of Sainsbury's yesterday evening. None of the staffed checkouts were open, so I went to the self-checkouts. On the way out of them they have fitted gates which don't open until you scan your receipt.
If there was a person barring my exit from a shop until I showed them my receipt, I'd feel insulted by what was effectively an accusation of theft. Having a machine perform the same role felt similarly insulting.
I appreciate that retail theft has increased, but treating every one of your customers as if they're a thief until they prove otherwise is not the right thing to do! (also I expect most theft at self-checkouts happens by people who do still check something out therefore have a receipt).