All we're talking about here is an occasional instance of a long-distance train. Long-distance Amtrak trains by definition don't worry about schedule, and if it's a planned detour there's even less concern. Plus: leaving a set of engines sitting out in the desert for much of the day (or more, depending on how late #4 is) is a waste of power that's needed for other things (Amtrak doesn't have much power to spare), would likely result in vandalism, would still involve a wye move (though just the locomotives), and would incur storage charges from UP. Also sitting there idling for hours burns fuel that isn't cheap. Bottom line: more cost-effective to wye the train, since there's a wye with a long enough tail track to do it, and who cares about the schedule which is busted with the detour already.

Consider it an occasion for listening to some interesting radio chatter, and if you're in the area some unusual moves to observe. If you're a passenger, you get some rare miles and a bit of jostling around as the switching is done.