Well I suppose. You want to eject the bomb too. So the spring would need to be under the floor. And then there's the possiblity the terrorist simply hold on to the toilet seat.
Encasing the alleged terrorist in the toilet though does allow him to be traced whereas if you just eject him and not the toilet, the terrorist could ditch the parachute and escape.
The downside of the ejectable toilet idea, of course is what happens if more than one terrorist is targeting the same flight. The loss of two or more toilets on a poorly ventilated airplane could create a terror flight of unknown proportions, not to mention a substantial biohazard.