Only that it's part of the membership agreement, and people choose to become members there, agreeing ahead of time to the terms (such as participating in the verification process at the door.)
That said, the employee violated probably every rule in the book by touching the customer, and should have simply notified store management or security to handle the matter.
Amazing how many folks these days will surrender their rights to government or corporations for saving money. One store tried to look in my bags once. I don't make eye contact with them. I pick a line close to the door. I have my receipt visible in my hand. I walk out keeping the same pace till I reach my vehicle. Only once was I chased and it was by a kid whom I verbally set straight on the law real fast then said have your manager explain it to you or a cop. I told him plainly either you have evidence I stole and will call a cop right now so I can sue for false arrest or you drop it and I'm leaving.
And BTW people never ever allow a loss prevention employee or manager to take you anywhere or go anywhere to another room with them. Stay put till the cops get there.
Another reason I don’t shop there. Pain in the rear policies.