Why do some UA agents not book PNRs that include other airlines?
 

Quite often these days, I seem to end up with itineraries that involve one or more flights on UA metal and then a flight at the end on another carrier for which there is no codeshare.

United (and most third-party engines) can price these mixed itineraries. But about half the time the agent at the 1k desk won't allow me to pay and book them. They usually say "that's not allowed," but the very first time I did this, an agent did it for me, so I knew subsequently that it was possible. Since then I have found that roughly half the time an agent will put it through for me. So I have just taken to putting the itinerary on hold and then dialing and redialing until I get someone who will do it.

Why are they so averse to doing this in the first place?