Cops pull people over for having something hanging from their rearview mirror. I would think they'd love to be ticketing people for having these things.
It's a federal emission standard though, not a state one. It's only illegal to do in New Jersey, Maryland and Maine. Every other state once you own it, you can install these devices, or do a EGR delete entirely if you want to. In states that do emission testing on private vehicles you'd want it to be reversible of course.
I don't speak German but I'm guessing they're ticketing/impounding vehicles that have modifications which violate emissions. Sometimes US cops to that to street racers... the problem is that's because the average officer doesn't like kids who modify Asian cars but they basically are the same demographic who would modify a truck.
As long as we're talking about that, the street racing and 'sideshow' thing is pretty out of control too. I'd be happy to see them step up enforcement on that bullshit.
States and local PDs can decide on policies, ie which crimes they want to arrest and which they will let the federal government enforce. For example some states have a policy of not enforcing federal marijuana laws. But this is just a matter of efficient use of resources. They always could arrest someone for a federal crime.
One important note regarding recent news from Texas: even if state police can arrest someone for a federal crime, that doesn't necessarily mean that they can enforce other federal laws, like immigration laws.