It is actually Californian state law (and other states that have followed their example) that requires this, there are laws now over modifications to any component that effects emissions which could include heads, cams, injectors, forced inductions, exhausts, headers etc etc. It also has laws that prohibit modifications to any safety devices fitted to the car which in theory could also include bumpers or any body component. Basically it's easy for the vendors to just state that the parts sold are for off road or race use only that way it pushes the responsibility onto the owner/driver of the vehicle.
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