It is not a state law, it is a Federal regulation under 49 CFR § 571.138 [1]. These requirements were fully effective for manufacturers and "alterers" as of 1 Sep 2008. However, yes, if your state doesn't require it (or doesn't require any inspection), then there's no way to penalize you as vehicle owner or reseller. Mechanics and aftermarket part sellers can still run afoul of USDOT though.

[1]: https://www.law.cornell.edu/cfr/text/49/571.138