The Federal Government set the engine RPM specification according to the length of the cutting blade for operator safety. It is called "Blade tip speed". A discussion can be found here what blade tip speed | Lawn Mower Forum and more info doing a Google search, if your interested. The blade is a highly rotating piece of steel that can cause severe injuries or death if pushed beyond it's limits. As you said, it is just a push mower, not a pit bike.