One simple point to make, there is no point in confiscating any sharp objects from passengers while glass bottles are sold in duty free.
Ask any casualty Doctor or Nurse if a broken bottle can be used as an offensive weapon.
I forgot BAA regulates security, BAA makes a massive profit from duty free. The alcohol and perfume suppliers will have said they can only redistribute their product in plastic if they increase the price at which they sell it to BAA, thus reducing the profit margin.
Nothing, but nothing, must get in the way of profit.