One motive is that it's cheaper to delete the emission system vs having to replace/repair the unit. Diesels have much more sophisticated emission devices vs gasoline engines, so it's more costly.
Many states and localities have "visible emissions" rules, where even if your vehicle doesn't have to pass emissions tests, you can still get pulled and cited for actual smoke. I've never been pulled for it (smoke means you're wasting fuel) so I don't know if it's an inconsequential fine or just not enforced generally.