How did the circus trainers mistreat Ruby, and why?

The circus trainers kept Ruby's feet chained to the floor, so she couldn't move. Their intention? They wanted to break her spirit.

“The circus trainers chained her to the floor, Ivan. All four feet. Twenty-three hours a day.”
I puzzle over why this would be a good idea. I always try to give humans the benefit of the doubt.
“Why would they do that?” I finally ask.
“To break her spirit,” Stella says. “So she could learn to balance on a pedestal. So she could stand on her hind legs. So a dog could jump on her back while she walked in mindless circles.”
I hear her tired voice and think of all the tricks Stella has learned.