Because if they are riding it safely, there would be no need to bypass the restrictions.
There is reason for speed limits, they are not there to f--k with people for fun.
That implies that speed itself is unsafe. Which it is not. Airplanes go some 500 mph and mostly don't kill people.
You can remove the speed limit of a vehicle for when you want to have fun on a closed course. You don't have to always go over the factory limit.