I can't answer for Subaru of course, but only the engine oil is subject to combustion byproducts, unburned fuel (while warming up) and any possible coolant leakage, and also the greatest heat (the underside of the piston crowns and the exhaust valve stems). It's the most hostile environment for oil, which is why it has the shortest change intervals, even before the 2008 turbo schedule changes.
Sorry, my intent was not to compare the fluid change interval requirements of the tranny/diffs to engine oil, but to point out that SOA has at one extreme; a theoretically "indefinite" interval requirement for tranny/diffs, and a rather short 3.7K mile required interval for turbo engine oil (both compared to industry standards).
I do not know of any other manufacturer that does not list a discrete tranny/diff fluid change interval for normal service (be it 15k, 30k, or 60k). Which, as I understand it, could allow an owner, while following all guidelines, and even having his services performed at a Subaru dealership, to never having the fluid replaced in his transmission - based solely on the judgment of the random service mechanic assigned to the couple of inspections throughout a 120k mile life.
I just find it odd that SOA would allow a situation where someone could a) have their car in for 60k service (all services performed to date but no tranny fluid yet changed) b) get a mechanic who happened to have a cold, so he didn't notice the burnt aroma of the tranny fluid, and then c) suffer tranny failure at 70k miles - just out of the powertrain warranty.
The owner did everything right, yet he has little recourse for his failed transmission