WW
While they have no authority to demand that a customer show said receipt, does not mean that they will not request a customer to show said receipt. You as a customer can either comply by showing the receipt and being on you're merry way or you may chose non compliance to show said receipt and therefore may be suspected of stealing and subjecting yourself to be trespassed. Also keep in mind that a private entity( think Casinos) can label you an " undesirable" person, and simply bar you from their property. Private entities that cater to the general public are not held to any Constitutional laws only Governments can violate you're Constitutional rights.
However if you feel the private entity is violating you're civil rights in some way, IE, they are only requesting black folks or Asian folks or white folks with long hair and beards or folks over 6 feet tall etc, to show said receipt, then you may have a cause of action for a civil rights violation.
Think sporting venues, concerts, movie theaters, all private entities that cater to the general public. They all have rules and regulations that they try to enforce and that customers and ticket holders adhere thereof. If you don't approve of the rules and regulations, you have a right to not purchase a ticket.
To quote Michael Corleone," It's business, not personal".
If you feel you have a civil rights beef against any company please seek proper legal counsel.
Regards
CCJ