That depends who you ask to some extent. It was the change from mainly rural chicken keepers to the quarter acre/whatever the US standard was/is if they have one. The post you linked to is part of it. Chicken keeping became an urban hobby. It supports a quite a few small businesses as well as adding another market for the large commercial suppliers.
So, I see the American model as urban, probably hen only groups, kept permenantly confined in a coop and run. The supply of hens is primarily through hatcheries with prefered breeders.
This is the core model for BYC.
That's partly why the Coop building and coop pages are so valued by BYC.
Much of the advice on BYC is biased with that model in mind. In most cases it doesn't matter because not many farmers take their chickens to the vet for example.
The poll came about after a bit of a debate between me and U_Stromcrow. It was about the so called 10% treat rule which I said was nonsense for many chicken keepers. My reasoning being, that even in a couple of hours of say supervised free ranging, the hens in particular will eat a lot of stuff, grass, bugs, grit, etc etc. I've seem hens go from crop empty to crop full in an hour on good forage. Compost heaps speed the process up. This is on a morning feed.
I estimated by volume that a hen that packs her crop from empty to full, three times a day will have ingested the equivalent by volume of 100grams of commercial feed. Also, it is unlikely she will have consumed nutritionaly complete forage by eating one selection.
If the above crop volume estimate is near enough then its quite possible for a hen to fill her crop in a couple of hours on reasonable forage. Not a lot of point trying to apply the 10% treat rule to that hen given she's just eaten 33.3... of her intake by volume on non commercial balanced feed.
U_Stormcrow stated he was giving advice for what he believed were the majority and that majority was the fully confined coop and run model.
I didn't think and hadn't thought for some time that the permanently confined coop and run model represent the majority on BYC. The poll is U_Stormcrows effort to put some data behind the two views.