Oh it's illegal. The criteria state the car can only bear those plates if normal size plates don't fit. In this case it's possible the rear needs a small plate. The Police and other government outlets have started penalizing for this, and technically, if a car has these plates, but doesn't need them, you can be sentenced to jail time for false testimonies. Or given a properly large ticket and forced to get normal plates.
I would've thought this would be stopped at the issuing office, not the police.
In any case, this car has enough space for European plates, even if it's a US-spec: