I can only speak for myself as an Amazon fulfillment center employee... I haven't directly witnessed any of the horror stories people tell about working there, but knowing the culture as I do, I would not be surprised at all that such things happen. Someone complained on the VOA board where I work about having been docked for time off task when they had to clean up after their own nosebleed, but that's anecdotal. You read about people having to pee in bottles to make quota, people passing out from heat exhaustion, people being forced to stand on injured feet because Amazon has a policy against employees sitting for any reason. Enough anecdotes pile up, though, and it smells like data.
The thing is, not everyone working at Amazon is a young, single person without any ties, who can afford to simply quit at a moment's notice and not worry about the risk of unemployment, or losing healthcare. So it's not a binary situation where if Amazon's employees are willing to work there, then clearly they must be willing to accept anything the company does because otherwise they would quit. For low income people, the cost of quitting can be significant, much more so if they have families to support.
Which is why regulations are necessary - because the relationship between employer and employee is not equal, as is often assumed in a perfectly ideal free market model. The ability of a company to coerce their employee due to holding arbitrary power over that employee's livelihood must be limited somehow, and there must be some means by which consequences can result for bad actors.
In an unregulated market, there are no bad actors, only inefficient ones. That's not a game most people can win alone against a billion dollar corporation.