Every Workers' compensation insurance company hires private investigators to make sure the person is really injured and not working another job while collecting. Sounds like the private investigative company overstepped their bounds by physically assaulting the worker, but otherwise this is SOP.
Worth noting - Amazon has nothing to do with this. They simply pay premiums for Sedgewick to administer the claims. The employer typically doesn't tell the insurer to go after the workers, since in this case it is Sedgewick who's footing the bill.