The practice started most likely during the depression era. When I was a kid growin up, my family lived in a two room house with a tin roof, and no plumbing. TV was unheard of, we didn't have a car, but Dad did have a 14 year old paint horse named appropriately "Ole Paint". If we were able to hitch a ride to town with Grandpa, (which we most always were), we rode in a old Chevrolet car, or if the roads were too muddy, Dad would tie two tow sacks together, throw them across the saddle horn and ride the 8 miles to town. We raised, and killed everything we ate, except flour, corn meal, and salt and pepper.
The reason the small store sold Dad and Grandpa 22 and shotgun shells by the piece was, a dollar bill was as big as a saddle blanket at our place, and we couldn't afford a whole box. Most of the farmers in our area were the same, and the store owner was a good man, that was not only makin his livin sellin, but livin amoung us, and we all helped each other. Sellin those shells like that was helpin someone that needed an occasional break.
Back then a 22 rifle (in our case a lever action) put hogs on the hook, as well as beef, occasionally squirrel, and rabbits, and when possible a skillet shot (ground shot at a covey of quail) with a shotgun might net 5 or 6 quail with one shot. A 22 back then was as valuable as many think their latest super duper gotta-have-it magnum is today. My Grandpa never owned but one lever action 22 rifle, and one single shot break-down shotgun in 12 gauge. It was all he could afford, and all he needed, and he never knew the difference or cared.