I agree with those who support the receipt/bag checks. I get receipts checked at Sams, Costco, Walmart and Best Buy, among others, all the time. Assuming it is a search under the Constitution, it isn’t unreasonable, to my mind. There’s no expectation of privacy in goods just purchased from a store.
But the the main thing I see here is someone just looking to be upset, and to have family members upset. This guy seems to have WANTED to be harmed. He wasn’t.
This isn't a Constitutional issue, it is basic contract law. The store has a right to set whatever terms and conditions it deems appropriate in the sale of its merchandise. They should have (and I suspect most do) signage indicating that all sales are subject to verification by receipt on exit. The potential customer can either accept the policy or go elsewhere. If they refuse the store's conditions at the exit door then no contract has been formed and executed and the goods have been removed from the premises without authorization despite having been paid for.
Payment is only one of the elements of a contract and all must be met before a sale is final.