I think your general point is reasonable, but far too centered on emissions. There are other reasons to limit modifications beyond just emissions. As an example, most turbocharged vehicles are de-tuned to a much lower psi of boost. The can run at much higher HP but also reduce emissions (depending on what type of emissions you're concerned with. Sometimes decreasing one type corresponds to an increase in another). They often force consumers to a lower psi rating for reliability reasons. So should a user be forced into higher reliability by a manufacturer? That seems like a much less clear answer than being forced into lower emissions.