Go back and check out the letter, y'all. The writer didn't say Grandma is eating things AND NOT paying for them, but eating things BEFORE paying for them, which implies to me that Grandma is presenting the empty packages (or produce stickers) and paying for them before they leave the store. I can think of several occasions when I was a kid and my parents would get a box of circus animal crackers, let me eat them while I was sitting in the cart, and then they would just let the cashier ring up the empty box with the rest of the groceries. If this is what's happening and the daughter in law is just embarrassed by what she considers to be uncouth behavior, maybe Grandma needs to find someone else to take her shopping.

"Before" might not be as specific wording as "without," but it doesn't necessarily indicate that they will eventually be paid for, just that they haven't been paid for yet. And, reading carefull as you encourage, the things we're talking about are candy, fruit,and bulk items. The last two have to be weighed. There is no package to hand over. How do you think she's managing that?

if you pluck a handful of grapes from the produce aisle, and pop them in your mouth, when do they get bagged and weighed and paid for?

Some grocery stores (I'm thinking Whole Foods here) prints out bar code scan stickers for items when you weigh them (so the cashier doesn't have to)

No, she's diving into the bins and grabbing handsful of stuff and yes, munching on grapes etc

Yes, the OP wrote "before paying" but exactly how do you pay for random amounts of food already eaten (aka grazing) once at the checkout?

No the letter doesn't directly state that they didn't pay. It says so indirectly. If the letter is correct and sincere, it's certain that the mother in law did not pay for the items. If she did pay for them, then it would mean that when they reached checkout, the old woman would have told the cashier that she'd eaten X amount of candy, and what kind it was, and then the store would have had to address the issue in some way. In other words, the store would have had to let the letter-writer and her mother in law know, then and there, whether it's acceptable or not. The fact that the letter-writer considers it stealing and is afraid they'll be nabbed by security on a future trip means that never happened. If the store had let them pay for it, then she would know Grandma is right and that it actually is ok, because the store knows about it and allows it. Of course she wouldn't tell the old woman that it's stealing if the old woman had paid for what she took.