Re: [B]why haven't the 2 stroke kit designers thought of this?[/B]
I agree that the idea has potential.
My guess would be that with the goal being cheap....CHEAP...it is cheaper and easier to provide 9 bolts, 2 hunks of rubber and a sprocket than to provide a front free wheel, plus that way fits more bikes. Also, you can't have a front derraileur. One last thing, the jackshaft in the SBP provides additional reduction because the standard gears on the rear of a bike all have less teeth than a typical driven sprocket. More reduction than the engine normally has is necessary before you can just drive the standard gears.
However, it can be done.