Like it or not, the store must operate within the LAW. If the law says you cannot FORCE someone to show a receipt for a purchase before leaving a store, you cannot do it.
You can ask, but you cannot force. I hate walmart for this very reason, and for others.
You’ve got your money, at this point I OWN everything in my bag, I have no legal obligation to PROVE to you I own it, and you have no legal right to harrass me as I leave your store.
I completely understand their need to do this, and have no problem at all in cooperating. I don't see it as an infringement on my "rights" since I choose to shop where I shop. It's nice knowing that the prices I pay there don't include the cost of them having to write off lots due to theft.
Making a stink over this is just plain ignorant.