Since it is not self-ticketing, i.e., where WN tickets WN employees to travel on UA and vv, then it's not a ZED agreement in the truest sense. It's an agreement where both carriers have agreed to use zonal fares similar or equal to ZED fares. Therefore, the ZED rules don't really apply.

UA is free to do what they want unless your staff travel office has specifically negotiated that they not collect fees other than the ZED fare + government/airport taxes/fees/charges.