They kind of treat components like cars when it comes to emissions. Or they're trying to. An exhaust does nothing on its own, therefore you'd think it's not subject to separate environmental regulation. However, they've been trying to stop people from selling such on the basis that once fitted to a vehicle said vehicle would necessarily be out of compliance since untested. The only way to comply would be to modify and certify every vehicle that you purport to sell the exhaust for.