Most readers have a metal core (onto which the electronics and display are mounted). Only the outer shell is plastic.
Metal does not protect too well against torsion either. You can only do so much for something that's thin and flat, and even metal bends. Hopefully the problem will vanish with the new flexible displays which new readers appear to be start using now (Kobo Glo advertizes the display not being made of glass, hope it's true, we'll see soon enough)