I think I've read that in another earlier post or possibly in the Dodge literature.
If it is correct, I'm not sure, it is probably due to concern about having the engine running, even revved up to 1500 rpm, and someone standing outside and reaching into the cab and accidentally hitting the shifter and jamming it into gear. Everyone knows the parking brake won't hold back the mighty Cummins.
Lot's of decisions are made by the lawyers on the staff of the manufacturers because of fear of lawsuits. The stupid reverse gear, door open alarm on the '06s because some dummies sued Daimler-Chrysler is an example.
Harvey