I can offer an experience from another vehicle platform, that might not be FCA's specific reason but could shed some light on it.
My other vehicle is a Mitsubishi Montero which came from the factory with an air locker in the rear diff. Like the Power Wagon it can only be engaged when the conditions are right according to the controller.
But some people have installed those lockers from junkyard vehicles into base model Monteros without the controller, just an on/off switch so they could engage them whenever they want. This has led a couple inattentive owners to drive long distances at highway speeds with the locker engaged and in both cases they ended up with heat problems. One of them had a tire blow out and he said both rear tires were extremely hot when he stopped the vehicle. The other had his pinion seal fail and belch out all the diff oil - my guess is that was probably heat related too because pinion seals generally don't fail over a period of an hour or two at normal temperatures they start leaking over a period of months or years. It seems to me like a locked diff at highway speeds produces a ton of heat, probably from the left and right side continually fighting each other over little things like a slight difference in tire pressure or the normal curves almost every road has.
The obvious answer here is "Don't be a dumb***" but from the perspective of the manufacturer they can't predict who's going to drive the vehicle - safer for them to foolproof it.
Last edited by spamPALA; 06-24-2019 at 01:39 PM.