It's probably programmed to shut off after a certain period of time for safety reasons. If the vehicle is parked in a garage that is attached to or part of your home, you would have a risk of carbon monoxide poisoning. Since carbon monoxide is an odorless gas, you'd have no way of knowing that your home was filling up with it unless you have a functioning carbon monoxide detector.
Yes, there are variables that could mitigate the potential hazard such as you're parking outside or in a detached garage. But GM has no way of knowing your particular situation, and could be programming the vehicle for worse case scenario (id. on the side of caution).
2018 Terrain Denali, 2.0T, Blue Steel Metalic/Black
2013 ATS Premium, 3.6 V6, White Diamond Tri-coat/Brownstone
1967 Camaro, 327, Butternut Yellow/Black