Legally, cars are not industrial equipment. One of the key distinctions appears to be that a hobbyist can modify a car whereas a hobbyist isn't supposed to be modifying cranes, tractors, trains, or the like.

Exactly - the government doesn't go after Subaru when people remove their catalytic converter because the car isn't industrial equipment. The EPA does go after tractor manufacturers and distributors when it is possible to modify a tractor to remove its diesel particulate filters and similar.

https://www.epa.gov/enforcement/tractor-supply-company-inc-s...

https://www.justice.gov/archive/opa/pr/1998/June/281.html

> The illegal vehicles had adjustable carburetors that were not described in their application for certification, were produced by a different manufacturer than the one specified in the application, were manufactured prior to the date of the certificate of conformity, had model names that were not identified on the certificate of conformity, or were significantly more powerful than described. The illegal engines were incorrectly certified as non-road engines rather than as recreational vehicles and some were significantly more powerful than described in the allegedly applicable certificate of conformity. The EPA and the U.S. Department of Justice also alleged that the emission control information labels on certain vehicles did not comply with federal regulations, and that Tractor Supply Company provided an incomplete and inaccurate response to EPA’s information request.

> According to the charges, the company's engine software controls the timing of fuel injection into the combustion chamber, causing the engine to emit excessive amounts of NOx while the truck is running on the open road. However, the company's engine software is designed in such a way so that these emission levels do not show up on the federal test.

What do any of these have to do with end users modifying their equipment?