this has happened on most of my flights. i think its about security. i dont think it has to do with snipers. the windows are double pane, i doubt a bullet would be able to get through both of them. plus, the change in air pressure from a hole in the window would cause problems i believe. additionally, if the plane was hijacked- the FAs likely wouldn't be allowed to give orders to the pax. plus, i don't think a hijacking would occur while the plane was still on the ground, as while on the ground, the plane could probably be stopped. and, nobodys going to hijack a plane during landing.