Do you have an integrated brake controller on your truck? If so it might be programmed that way since you don't really need the brakes to activate when parked.
Aftermarket controllers, at least the ones I'm familiar with, don't have any way of recognizing what gear you're in so should still activate the brakes in park.
2011 Chevy Silverado Crew Cab 4X4
2009 210 WBS