Covering the additional labor to process the UMNR and documentation and chain of custody.
Risk management particularly if something were to go wrong inflight. E.g. If UA diverts for whatever reason UA policy requires an employee to remain with them, including to the hotel if overnight lodging is required.

Highly doubtful. UA doesn't really want to be in the UMNR business anyway, and an agent waiving a UMNR would expose themselves to employment risk.