I agree with everything you say. There is no need to be rude to the greeter. A simple "no thank you" and keep walking. Where Wal*Mart would get into trouble is IF they don't accept the "no", and stop the customer from leaving the store because of the refusal to show the receipt. And yes, it's happened. A simply Youtube search will find a bunch of videos.
Now, IMO, if a) a customer does not have to stop and show a receipt b) they're not comparing the receipt to what's in the cart/bag, I have to ask "what's the point"? Of course, they're a business and they can have employees do what they want (for the most part).