That’s exactly what they depend on, along with “he must be guilty, he is in the dock”
while it is perfectly legal to ask to check the bag, they have no obligation. They cannot hold you with no evidence, but they can ban you from the shop. Although there is implied right of access, the shop can post up terms and conditions, presumably they have to be in a prominent position.
I was a member of Costco and they stop all customers at the gate. I am no longer in Costco, but not because of that. Presumably (never read them) those terms are in the joining Terms?
I frequently go to trade shows where there are also people selling goods. I do find it a pain that they insist on bag searches and so now don’t take a bag, or buy anything. I make a point of telling the stall holders when I collect their flyers the reason for this.
it does seem ironic that they save money by introducing self checkout, then have guards searching bags.