That's a good question, and I would have to say that they did it out of sheer ignorance and convenience. They are the type of people who kind of look at animals as something that will serve them, and chickens aren't pets. I think it was easier and cheaper for them to feed the chickens all leftovers in order to cut down on buying actual chicken feed. They wouldn't harm the chickens intentionally, but they definitely aren't newbies either...you'd think they would have learned by now not to feed the chickens Fruity Pebbles and Doritos for dinner.
Edit to add: I just remembered, this certain neighbor would also sell her eggs to other neighbors, and if asked, she would say they were "organic"Again, I chalk it up to ignorance. She wasn't trying to lie, she really thought that since her hens laid them in her yard, that made them organic.
I think on this we probably should remember that it's what all backyard chicken keepers used to do before manufactured feed was available to the public, and I know at least around here where I live there's still alot of "oldschool" people that think what they did back then is still as good today. Ofcourse, they didn't have Fruity Pebbles and Doritos back then. They didn't have fridges for stuff to be pushed to the back and be forgotten about. But the chickens really did get everything that wasn't considered fit for humans. Pretty sure they didn't get penned up, either, though.
We have better options now, and as the stewards of those that depend on us, it's our responsibility to provide them with the better options at our disposal. But the mentality still exists that what worked then still works now. I've met quite a few people in my area that pen up their birds to protect them from predators, and feed only scratch and table scraps, and think it's normal to only get 4 eggs a day out of 14 hens. I've tried talking to them, and they just shrug their shoulders. We also have a ton of people in this area that keep horses year round in 15 x 15' pens with no shelter and standing in 2 feet of their own dung. Pretty sure they didn't do that 150 years ago.... So, there's that.