The bookstore at the school I just graduated from checks receipts at the door. Last semester, I paid for my books, put them in my backpack, and headed out. The guy at the door asks me to stop so that he can look in my backpack. I told them that there was absolutely no way he was going through my personal property.
He continued to insist, and then decided to get a manager. I almost left the store right then, but decided that I could spend a minute to talk to the manager too. The manager gave the same story about how all bags need to be checked as we leave. I told him that unless they had a reason to suspect that I had stolen something, that they had zero right to inspect my belongings or attempt to keep me from leaving. I asked if he would like to call the police to sort it out.
He decided that calling the police wasn't necessary, but told me that the polite thing to do would be to comply with their request. I told him that the polite thing to do would be to not treat their customers like criminals, and walked out.