NBCU has contracts with preferred vendors (see Concur for listing), that is based on volume commitments.  All air travel must be booked on these carriers. United Airlines is not a preferred vendor.  Exceptions will be allowed if the cost of the ticket is lower than others offered at the same time in the same class of service; or if there is a business exception which must be approved by NBCU Travel ([email protected]) such as it is the only flight available for a particular destination or is the only non-stop service available.