I fully agree with you and recognize that a person can only become the way I am (and doesn’t necessarily) by growing up in a privileged environment where store owners are always happy to see you and your family and you have never been questioned by anyone.

If you feel that “groceries aren't mine until I've paid for them,” then how do you ever eat at a restaurant?

Hmm, touché :)

I think a restaurant is different though because that is explicitly how it works (eat first then pay).

To take your comparison further, what would happen if you sat down in a grocery store and started consuming hundreds of dollars worth of food and drink? I think it would raise a lot of eyebrows, and the employees would probably confront you. So there is a difference.

Furthermore, in a restaurant you place an order and they serve you, and from a legal perspective (IANAL) there might be something about some sort of implicit contract that you will pay them. Whereas with a grocery store you are not allowed to remove the items from the store until you've paid for them. And I feel like consuming the items inside of the store is sort of analogous to removing them from the store.

When I was young, I avoided restaurants because of exactly this! Always preferred cafes/fast-food places where you pay first. It just didn't make logical sense to me. I've grown out of it now obviously... :P

Doesn’t make sense. Paying beforehand is customary at grocery stores, not at restaurants.

I've never had unpaid-for groceries in a restaurant.