NOx is actually not an issue if you are not an a heavily populated area. And when I say heavily populated I mean millions. For example, San Antonio has never had high NOx level days to warrant any health issues until it reach a population of 2 million people and even thin it is only only select days in the summer when the temp is above 100F. In urban areas where it does not stagnate, NOx dissipates into the upper ozone where it is actually good for the environment since it is an ozone gas
So unless you live in these areas, your NOx output is not hurting anyone. However, since it is much harder for the EPA to control what vehicles get sold where, it is easier for them to just apply all standards to everyone even though people like me do not live in these big cities. Basically, I have to put up with these emissions controls simply because it is easier for the EPA to mandate the manufacturer than mandate the people. I say, force people to move away from big cities than force me to have these devices. That would actually be much better for the environment than some of the NOx limit rules. If you look at air pollution maps, cities that are more spread out have less air quality pollution.
FYI, NOx should not be confused with CO and CO2 which are mainly emitted by gas engines, not diesel. Since these do not dissipate like NOx, it doesn't matter where the are emitted to cause harm to the environment.