Okay I have a bit of background because I did my dissertation on collagen expression and mineralization. The animal model was the chicken.
The problem with fast growing chickens stems from the 1980s when white meat (breast) was all the rage. They bred birds for huge breast muscles and also for speedy growth. They were harvested at about 6 weeks of age.
Problem was their bones were still growing and they became so heavy that the young bones could not support their weight, leading to all sorts of skeletal problems. Because they would get broken bones they could not be sold.
However for the most part the chickens are not raised in inhumane conditions, especially the broilers. They live in a pen and are free to move about. I worked at three different research farms and the conditions for the animals were not bad at all.
My DIL was given a pair of broilers rescued from a farm that were so fat they could not get up the ramp into the coop. They just waddled around and made their nest underneath it.