Technically, you are correct, UNLESS the supermarket allow/encourage you do pick up items, inspect them, etc. This is implied and any employee will permit a customer to touch and inspect items before payment.
Now, if a supermarket explicitly states that customers may consume items before paying for them...then its fine. I used to work in a supermarket and at no stage have I ever seen a sign stating that customers are permitted to eat the food, before payment.
What the lady did by treating the supermarket as a buffet was not right. She is 100% in the wrong.
I can well understand if parents need to feed their brats before paying for the food, as some brats can cause chaos in a supermarket, but adults should know better and should show some will power.
Never in my life have I gone in to a sweet shop and started eating a chocolate bar before paying for it. Same goes for the supermarket.