There is no security advantage to turning them off, you might have other things to think about while being hijacked, and, in any event, the point is not that people should receive the exact same message. There's no need to find out via cell phone now that passenger planes are bombs -- we know that. It's the next important revelation, the one we don't know, that I'm talking about.
It's mainly the irony I'm pointing to. The government persists in thinking that their controlling of information will help, and transparency and open communications will help bad actors, when the opposite is demonstrably the case.