Many manufacturers are headed that way. Why are we building things that are held, dropped, put into pockets, tossed, etc out of glass again? Even on the surfaces that don't need to be.
I seem to recall Apple owns part of the company that manufactures the glass. It would be kind of dumb to not use it everywhere and anywhere at that point.
They probably looked at the numbers and realized 98% of people put cases on their phones anyway so the disadvantages of glass weren't too bad.