The way I see it, he is on their private property and it is common courtesy and ethics to respect the rules they have regarding their property and how transactions are conducted within it, such as confirming you aren’t leaving their property with stolen merchandise. He may have found one law to defend his belief, but just because something is lawful doesn’t make it ethical or respectful.
I may have a legal right to free speech, but I’m sure he wouldn’t appreciate my coming into his private property such as his house, and cursing him out in front of his children.
if the law says you are not allowed to ask for a reciept after payment is made, it does not matter if it is a private property.
this is no different than states that say a ccw permit means you can’t prohibit ccw in public invided stores.