Edit: to people wondering why I do this, I can only say that it’s something that I never even thought of as weird. That’s what growing up rich can do you.
I have a friend that does the same thing and it just seems really weird to me.
My groceries aren't mine until I've paid for them. And also, as a person with not always a lot of money I sometimes (very rarely, but still) find that I don't have enough money for the things I wanted to buy. What if I'd started consuming the goods, got to the till and couldn't pay?
If I needed to drink first I'd buy the drink first, drink it, and then proceed with the remainder of the shopping.
Anyway, people can do what they want. I'm not going to berate anyone for this. But I still think it's really weird.
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.
I would also add that if the store owner came up to you and challenged you to please pay for that right now, you're obligated to do so. Considered as a whole, it's not a good idea for someone to do that over a bottle of soda for all kinds of reasons, but I'd say the right still is essentially there.
Well you can hope, but it's made by humans, so I wouldn't count on it.