I don't know about any state except for Virginia, since that's where I lived when I was in high school and working as a supermarket cashier, but there it is illegal to charge for the weight of the bag (or any other container). You have to tell the computer how much of the weight is actually the container so that the customer does not get charged for it.
Although the cost of the bags are part of the operating costs of the store, and are thus figured into the store's prices.
How much would a product have to cost, per pound, before the weight of a bag would add a cent to the cost?
You may very well be right. I just figured that someone like Wally bought those bags at a buck-a-bale and sold them for whatever the contents cost per pound.