At Costco and Sam’s, you sign a membership agreement which I believe allows them to perform the search. At Best Buy, you can just walk on by the receipt checker, as you have entered no such agreement.
Still, they have you locked away from anything when you go through the register area. What is left for you to grab? The area is fenced off, so you cannot go back to the floor except through the register area or back through the front. What do they think you did, pick up a 50 inch TV?