OK I looked into it because I remembered that ram uses two different transfer cases, and this is the cause. They use the borg warner 44-44 and 44-45. Lets call it, 44-44 "auto" and 44-45 "locking".
No it cannot be installed afterwords, at least not simply, as the transfer cases for the 4x4 work differently and would need to be replaced.
The AUTO is in the higher trim models and is more of a convenience thing. Letting you put it in 4 auto so you dont need to mess with it. HOWEVER it is NOT as "good" of a transfer case for a situation when you really need 4 wheel drive. The reason they use the locking and not the auto is because the truck is intended for offroad. The AUTO transfer case uses a clutch like an LSD to engage and disengage the front portion of the drive train. IT DOES NOT ACTUALLY LOCK, EVER.
The one that comes in the lower trims, and the Rebel, DOES ACTUALLY LOCK into 4 wheel drive. This is why the outdoorsman comes with it too. The slipping clutch of the auto transfer case will eat its self up in heavy mud or snow. The "lower trim" does not since it actually locks and is locked the whole time.
If you want to see what I am talking about check out this video,
https://www.youtube.com/watch?v=gEhT-scgV2o
and this old thread that talks about it,
outdoorsman vs big horn differences - Page 2
So it looks like it was removed to provide a more robust vehicle and better performance. Which judging by the first reviews linked on here it does very very well. I bet the initial rams that they showed in detroit were just laramies that had the rebel interior put in. Its basically a laramie anyway. The ones at the chicago show missing the auto button were likely made later and were actually 2015 rebels.