Your car is only regulated by where you have it registered. So, if you are in Tennessee as an example, Davidson Co residents pay higher plate fee and also must pass inspection. Residents in Rutherford Co pay a slightly lower plate fee and no longer have to pass inspection. Inspections in the US are largely emissions. Some counties have dropped this because they found it ineffective or not cost efficient.
If your vehicle doesn't meet obvious road standards, you'll simply be pulled over and ticketed.
Interesting that it's done at the county level in TN. In my experience (which, to be fair, is mostly in the Northeast where counties are pretty much irrelevant) these laws are usually made at the state level, not the county level, though your point stands when comparing states.
Since cars typically drive on state funded highways it would make sense to package car inspections along with the "you better set your state drinking age to 21 or you'll lose all highway funding" bill.
I don't necessarily disagree. It is pretty wild if you think about how unregulated a vehicle is after the manufacturer hands it to the customer, but that's not how -most- states operate. I guess you could just focus on states that are higher population density and hope it just rolls downhill. However, a lot of people game the system by just registering in different states or counties...it's not new at all. That is very popular with plate fees, even in liberal states like California that use certain counties to avoid higher plate fees.
If your vehicle doesn't meet obvious road standards, you'll simply be pulled over and ticketed.