unscrew that bolt, remove the collar around it, and screw it back in and it helps pop off the rotors so your not stuck bashing a good rotor to get it off if its seized.

if you remove that bolt, the threads can rust and prevent future use.

 

Actually, that kind of problem (swapping rears for front) can completely throw off the handling of the G. By preventing the rears from fitting on the fronts it won't allow the VDC to get confused by offsets.

Idiot bolt? Not really, just a simple, cheap device used to prevent issues.