It may sound harsh, but I consider the act of opening or consuming a product that is still technically the "shops property" stealing. You have not yet paid for the item even if you have the intention of paying for it, its theft as it still belongs to the store.
I agree. When you work at a supermarket you have to get special stickers put onto anything you take into the store that you can buy to make sure you're not taking things off the shelf.

In a shop, there is the expectation that you buy before you eat also. In a restaurant, there is the promise of payment, you have informed the staff of your consumption by ordering the food. In a shop there is not that same promise.
Again, I agree. I think it is faulty logic to compare restaurants and supermarkets - They are not the same beast. There is a level of accountability at a restaurant when you consume food before paying. You have to put an order in and what you eat is recorded before you eat it and then re-recorded when you are given the bill. You are handed the food by staff and sit down in full view of them to eat it. It's a very different situation to helping yourself to things with no one knowing what you have taken and how much as you walk up and down aisles, relying on you being honest enough to present it at the counter when it comes time for payment. The only accountability you have there is what you place on yourself and maybe what the security camera's might hold you to when you're getting charged for shop lifting.

Those products are the shops until you purchase them to make them yours. Helping yourself to them before then is just rude in my opinion (you wouldn't go into someone's house and help yourself to their pantry). As for the excuse of "I'd rather that than a screaming child", I don't see why the shop has to take on the burden of responsibility either for the laziness of not taking something with you, the laziness of not going to the checkout to buy something up front at the beginning of the shop (and getting your sticker put on it) or simply teaching children delayed gratification.