Agency staff have a contract of employment with the agency who sells agency work to an employer, ZHC staff indeed cut the middle man out and have a contract with the employer but the contracts could be absolutely no different than had that person been employed full time.
Again, the treatment of people in employment has nothing to do with the type of contract.
It is not specifically written into a ZHC that exploitation is allowed because a ZHC contains the exact same workers rights and minimums that a full time one must have.
Conflict in terms of hours is the only possible avenue in a ZHC because it is the only difference.
My company has a lot of sub-contract staff and they were recently told that work has quietened so instead of doing 48hrs they could do a maximum of 37. Is this exploitative?