It has always been in our cultural DNA to "make-sure" and be more "kiasu" than necessary when dealing with matters of importance. But when dealing with national security, isn't being kiasu a good thing?
Granted it was "only" a small plane.. but the boat that damaged the warship USS Cole was pretty darn small too - only problem was that it was packed with explosives.
Yeah, traffic controllers could see the plane and identify it as only a little cessna plane but I'm not aware of any radar technology out there that can scan and identify what's inside the plane.
So what if the plane had been packed with explosives? Does that scenerio sound too impossible? Personally, I don't think so.
So why divert such a potentially explosive package to Changi? Well, my own thinking is that it is the only place that the plane can land without flying over any more of spore's land than necessary. If they had to shoot it down, at least most of the debris would prob fall into the sea.