I think we should detain Rick until he shows us the law. ;^)
I don’t think Rick was claiming Walmart was breaking the law by asking to see the receipt. I could legally ask strangers on the street for receipts for items they have as long as I don’t harass or detain them. Can I please see the receipt for the computer you are using now? See? nothing illegal. :-)
Physically preventing Rick from leaving the store without probable cause would be the violation of the law.