They can quit, but they aren’t likely to find work as easily as Amazon’s staffing agency can find a replacement or for Amazon to replace the staffing agency.
That is known to the individuals working at Amazon, the staffing agency, and Amazon itself. Absent inordinately good conditions, that can set up strange incentives to treat the staff with a certain degree of contempt and distrust.
For what incentives exist to set things up this way, I would be surprised to hear of genuine (read: not PR-driven) examples to the contrary. That is, somewhere that values their employees and agency help in ways that positively incentivize someone to want to work there (read: in ways other than it being the only thing available).
Again, I suggest that they quit if they don’t like working there. Spare the intellectualizing. If they can’t keep employees they will change their methods and practices.