Why don't combat aircraft have rear-facing *laser* weapons?
Such weapons are not used by countries that abide by the Geneva Convention:


  It is prohibited to employ laser weapons specifically designed, as their sole combat function or as one of their combat functions, to cause permanent blindness to unenhanced vision, that is to the naked eye or to the eye with corrective eyesight devices.


For the U.S.A., page 45 of Navy Shipboard Lasers for Surface, Air, and Missile Defense says: