UA agents certainly *can* do OAL rebookings.

UA may *choose* not to do OAL rebookings for irrops on award tickets if they *choose* to set and follow a policy that award tickets only get rebooked on UA or on Star Alliance carriers. I've certainly heard reports of agents refusing to re-book an award ticket outside of UA and citing "policy". Whatever their rule is, I haven't seen it publicly published anywhere.

(You can imagine why UA might set a rule like this -- revenue tickets actually make UA money and help offset the cost of paying another airline to transport you, while the form of payment for award tickets merely reduces a liability to UA and doesn't actually put any cash in its pockets.)

If you review the contract of carriage I think you'll find that UA is only obliged to get you to your destination, not to buy you a ticket on the next available flight to your destination. (Double-check -- if I remember right, they may actually only be obliged to get you there or to give you a refund?!. Although I think they probably wouldn't want to refund your ticket and strand you at ICN, because they might end up legally responsible for transporting you back to SFO if visa requirements meant you couldn't stay in ICN indefinitely.)

In this case it's a bit silly that they didn't rebook you on EVA — EVA Airways is not a *A member but they are United partner and you can book award tickets on EVA with MileagePlus miles!