the relevant laws: us code title 40 chapter 1 covers everthing you need to know.
section 168 defines the requirements of testing for aftermarket exhaust systems.
it is the manufacturers requirement to meet epa standards BEFORE selling them in the US markets.
follow this up with us code title 42 section 4905
(d) Warranty by manufacturer of conformity of product with regulations; transfer of cost obligation from manufacturer to dealer prohibited
(1) On and after the effective date of any regulation prescribed under subsection (a) or (b) of this section, the manufacturer of each new product to which such regulation applies shall warrant to the ultimate purchaser and each subsequent purchaser that such product is designed, built, and equipped so as to conform at the time of sale with such regulation.
when combined with
(e) State and local regulations
(1) No State or political subdivision thereof may adopt or enforce—
(A) with respect to any new product for which a regulation has been prescribed by the Administrator under this section, any law or regulation which sets a limit on noise emissions from such new product and which is not identical to such regulation of the Administrator; or
(B) with respect to any component incorporated into such new product by the manufacturer of such product, any law or regulation setting a limit on noise emissions from such component when so incorporated.
It effectively states that the manufacture of the exhaust is liable for meeting epa regulations and that they must warrant their product as such and are responsible for all claims against those warranties.
in a nutshell if they are sold by a US manufacturer in the US then the manufacturer is either breaking the law and liable for all claims against the product. or has complied with the regulations and any state/locality claiming otherwise is violating federal law.
can we all move on now.