Just thinking to myself the diff in braking between disc and drum I can possibly understand the warranty issue.
Drums usually aren't set so tight they can lock up or your trucks controller won't put out enough current to fully apply the drums to the lock up point whereas disc brakes could possibly be able to put more braking stress on the chassis or frame/spring mounts because of being able to lock up if not set properly. After all that's why people like disc brakes because of the better stopping power. Just a thought.