sync navigation interface enabling

Why can't the SYNC system use the passenger seat occupancy sensor to allow the navigation (and other) system(s) to be usable when it detects a passenger is present ? I find the fact the the navigation system is partially disabled but the other interfaces that can be as distracting (i.e phone interface) are still operational ??
glenn b 02/02/2013
This would be tough to retrofit, but see Ford Navigation for Passenger suggestion. A second paired phone would be sufficient verification there is a passenger to operate the navigation system.