DoT Rule 240 -- disappeared with the ADA in 1978.
In UA's contact of carriage, RULE 24 FLIGHT DELAYS/CANCELLATIONS/AIRCRAFT CHANGES and really has nothing to do with diversions.
However when legacy airlines stuffs an inability to provide transportatiom they will will book on OAL and UA will do that as you learned (but typically you need to ask - as you did, the first preference to to find an UA provide resolution). Normally SOP UA will handle high status passengers first and full fare will likely get the same treatment. My guess given it takes some time and effort to push passengers to OAL, UA was not able to make use of all the spaces available in the short timeframe. No mysterious, hidden rules -- just standard processing of passengers in irrops in a limited time.
No mention of fare class in UA's CoC but common practice is to address your "better" cusomers first
When a Passenger’s ticket is affected because of a Schedule Irregularity caused by UA, UA will take the following measures:
(i) Transport the Passenger on its own flights, subject to availability, to the Destination, next Stopover point, or transfer point shown on its portion of the Ticket, without Stopover in the same class of service, at no additional cost to the Passenger; or
(ii) At its sole discretion, UA may arrange for the passenger to travel on another carrier. United may also, at its sole discretion, and if acceptable to the passenger, arrange for the passenger to travel via ground transportation.
Force Majeure Event - In the event of a Force Majeure Event, UA without notice, may cancel, terminate, divert, postpone, or delay any flight, right of carriage or reservations (whether or not confirmed) and determine if any departure or landing should be made, without any liability on the part of UA. UA may re-accommodate Passengers on another available UA flight or on another carrier or combination of carriers, or via ground transportation, or may refund any unused portions of the Ticket in the form of a travel certificate.