This is actually a good point. The purpose of the New York bus system isn't to emit four billion pounds of carbon dioxide, but its purpose also isn't just to transport New Yorkers.

Its purpose is to transport New Yorkers by emitting four billion pounds of carbon dioxide. They're choosing to do it that way instead of doing something else, and instead of doing nothing.

And then it's kind of hyperbolic rhetoric to claim the purpose is only the bad thing the system does, but it is actually doing the bad thing, so then if you want to claim that its purpose isn't to do that, maybe get it to stop doing that.