There have long been rumors floating around that at the time that UP took over SP, that there might have been an exception made to UP's rule for the 819 because of its location on the system, but that at least some of the persons in discussion with UP started the conversation "well this is how we did it on the SP and this is what we want UP to do" which was sort of a non starter in dealing the UP people.
As far as the "no foreign steam" I have been told by some people in UP that it is not the "foreign" issue so much as it is "not maintained by UP people in a UP shop" issue. That goes back to the liability issues and the "what happens if something breaks, and the steam train delays the important stuff". If it is yours, then you are responsible, if it is not yours.......
There are UP, SP,SSW, WP steam locomotives that are or have been operational in the past but they are not serviced and maintained in Cheyenne and I have been told that is the key.
Imagine what the results would have been if it had not been 844 involved in the incident where the diesel helper did not shut down, and the engine got flat spots on the drivers....
The policy is not totally monolithic either. Reader RR #2 operated on the passing siding at Granger, TX for the movie, True Grit. Of course it was not on the main line, and before it could be fired up it had to undergo an inspection by a UP steam crew boiler maker, and there had to be a UP pilot engineer in the cab any time that the locomotive was moved or moving etc. etc. And it had to be lettered Union Pacific (which you never saw in the movie), but it was live steam on UP even if not on the main.