Amazon has been incredibly hostile to labor rights for their employees, including worker safety, fair compensation, parental leave, provision of basic benefits, reasonable schedules, collective bargaining, etc. They've been cited by federal labor judges for systematic unlawful behavior, and are under congressional investigation. Hopefully there will be some real consequences sometime soon. I'm not holding my breath though.
In the markets for package delivery and warehouse logistics, Amazon's uncompetitive and worker-abusive business practices and basic design have been putting systematic pressure on other firms to similarly mistreat their own workers to compete. In the more general retail market, Amazon's unlawful (but thus far substantially unpunished) anti-competitive business practices drive smaller competitors out of business and extract rents not just from Amazon's own workers/contractors and suppliers but from the economy broadly.
> are free to choose to work for a different employer
This is certainly more true than in a feudal economy where peasants are literally considered the personal property of the lord, but in many of the relevant local labor markets Amazon is a disproportionately large and influential employer without enough local employer competition that employees can easily switch jobs. The only real way for employees to get a fair bargain in such situations is collective bargaining via a union, but Amazon is notoriously opposed to union formation at its facilities, which it blocks unlawfully wherever it can.
Amazon doesn't use systematic physical brutality or murder to maintain their power, as done by e.g. feudal lords or drug gangs, but they do routinely (unlawfully) threaten workers with loss of pay/benefits or termination when those workers exercise their legally protected rights, and unlawfully follow through on those threats when workers persist. Since most people need to collect a paycheck to survive, this kind of violence is not entirely dissimilar to more direct physical attack.