Does it really matter? Cart lady got there first, fair and square, and while it may be impolite of her to take a lot, it is significantly WORSE to steal out of somebody's cart.
Would you teach your children that it's okay to steal if somebody else has a lot of something? I doubt you would.
You're not being denied the chance to purchase anything just because it's all gone. Doesn't matter if one person took a ton or a ton of people took one. It's out of stock. If there are no limits on an item and no storewide household limits, a person is free to take what they wish. Perhaps cart lady wasn't even using coupons, it was just a good sale price for her.
She might not have a stockpile, but even if she did, who cares? You have a stockpile, I have a stockpile. Neither of us would be very pleased if we went to add to our stockpiles and somebody stole our purchases out of our carts, justifying perhaps that since we already have a stockpile at home they're more entitled to the item than we are, despite us getting that item first.
Regardless, if your desired item is out of stock it can be a pain in the @$$ but you can ask for a raincheck. It may be annoying for people who don't have a vehicle to have to go back at a later date, or to have your coupon expire, but that's life.
There is no reason in my mind that justifies stealing from another person.

