Why? Because your car is 40 times less safe than school bus? In fact, those in the back without seat belts are not only risking their own lives, but are also a deadly threat for the driver and a passenger in front. See http://www.youtube.com/watch?v=4TwkGMeQjTQ#t=4m (in Russian. The first test is with front with seatbelts, in the back only child is using it. The next test is front without seat belts, in the back both adults are using them, a child is not).

BTW, in my country seat belts are mandatory in both front and back of the vehicle.

Actually, the big reason to put on your belt in the rear of the car is that the mortality rate of the front seat passengers goes through the roof when the rear passenger does not have a belt on. The front seat passenger is held in place by a seatbelt and then pummeled from behind by the person in the rear seat slamming into the back of the front seat.

Because you are a small child and your car is as tall as a bus and the seats are designed to cushion impacts anyway?

Can you cite where you got this?

I imagine it'd be very easy to get ejected from the back seat of a car in a rollover/accident.

Do you ever sit in the middle back seat?

If your car had no seat belts in back, then it would have the 4" of foam on the back of the front seats, and you might be OK.

I can just hope that the people in the front seats realize that you are effectively risking their lives by your stupidity before it's too late.

And that is called irony.