Why does Onstar Turn by Turn Require ABS system installed?

For a car with on-star to have the Turn By Turn GPS they also require you to have ABS (anti-lock braking) installed. This seems like a strange requirement. I know some hardwired GPS units tie into the cars computer so when satellite lock is lost it can keep giving directions based on car speed, but this should required ABS. Anyone know what information a braking system can give a GPS unit that speedometers or engine output can not?|||The ABS system just give onstar a clearer reading when turning because ABS braking is based on turning. ABS braking is used in situations like turning where the wheel automatically brakes to prevent rollover or skidding by using sensors. Onstar uses this to accurately know when the car is turning in a 90 degree angle or a smaller angle where ABS braking wont be used such as 20 degrees. It is pretty advanced technology and uses many sensors and speeds up results and accuracy.