It should be legal to run a straight-pipe if your cat gets stolen through no fault of your own. The economics incentives of the black market are there in the first place largely proximal to policy. As it stands .gov gets to light the candle from both ends, going after those failing emissions while also going after the cat thieves.

Edit: Love the helpful comments though telling poor people just to move out of the city if their cat gets stolen, as if moving is easy if you can't afford to replace a cat.