"Rolling coal" remains an illegal modification, practiced by scofflaws. TÜVs are not immune to conflicts of interests.[0] And while generally not a separate sticker from annual car registrations, annual or biennial emissions testings are carried out by most states.[1]

>any sudden punishment of violators from outside reeks of market protection.

Perhaps it's because the EU lacks the authority of the member states to regulate & police emission laws. They started legal proceedings against their member states precisely because of their inaction.

"The European Commission on Thursday began the so-called "infringement procedures," calling Germany, Britain, Spain and Luxembourg to account for not imposing the same kind of penalties Volkswagen faced in the United States for using illegal software to mask health-harming emissions."[2]

Isn't that market protection as well?

[0]: https://www.nytimes.com/2015/10/15/business/international/co...

[1]: https://en.wikipedia.org/wiki/Vehicle_inspection_in_the_Unit...

[2]: http://www.dw.com/en/eu-launches-legal-case-against-germany-...