Puerto Vallarta is sooooooooooo much safer then many other destinations. There has been a killing (NOT a cruise-passenger), very unfortunate of course. But much more killings happen in other places (also against tourists!) and ships still sail there, tourists are still coming.
We have had various long winterstays in PVR, never had any problems. Not while riding buses, shopping, dining, riding our car, or even walking the streets during the day or night.
The decision of Princess has nothing to do with safety, otherwise they better stop sailing to L.A., San Diego, Miami, Seattle, Vancouver, Barcelona, Rome, Istanbul, Athens, Amsterdam, just to name a few places which are less safe.
Prices are low, most passengers donot book excursions, so Princess doesnot make enough money on the Mexican Riviera. Sailing not that south to PV but staying more north saves a lot in fuel.....