You're kidding, right?

The terrorists would then just not stay in their seats when they took over the plane - I doubt that they do now.

--

Ejector seats for the passengers would not be practical . First of all ejecting at 25,000 feet would land them in a frigid oxygen-deprived atmosphere. They would lack oxygen masks and parachutes, not to mention the training about how to eject safely and parachute without too much damage to themselves and others. Secondly, with all that 'open air ventilation' in the roof of the plane the pilot and other crew would be pretty hard pressed to continue flying the craft safely.

Not to mention that infants and toddlers, who are stapped into car seats for the ride would be shot out of the plane to their deaths - do you want to be responsible for that?

Not sure... but the thought gave me a serious case of the giggles... thanks.

Because the concept is totally impossible.....

I'd settle for just a parachute.

Well...
First: It's not practical: You have to build the plane so as to open the roof to let people eject, you have to incorporate parachutes in the seats, oxygen masks.
Second: It's not so safe to eject. Many militar pilots who ejected had severe spine injuries.
Third: Stop thinking about terrorist trying to hijack planes, they won't try it anymore...

Once the pilot decided to eject a possible terroist, the whole plane would explode, at 30,000 feet.

Not a good idea.