Did you eat crackers and snacks and drink pop at the grocery store, and give the cashier the empty box or bottle to scan?

It never, never crossed my mind to do that. We were in line behind a lady and her toddler at Walmart. The kid noticed I had bananas and she started saying “nannas.” We really bonded over those bananas! So I asked Mom if I could give her one after the cashier scanned them.
She said, “No. She’s not hungry. She munched on Nilla Wafers all though the store!”
The cashier had also scanned an empty Pepsi bottle.
That kind of blew my mind. I guess it’s acceptable but…why?