It seems to be relatively common to operate aircraft in Europe as N-registered.
Oldakro,
Firstly, all those N registered aircraft are normal FAR 23 or FAR 25 aircraft, not "Experimental" or "Limited" category --- without the express permission ( usually something which is, effectively, the same permit to fly as you would get for a local registered aircraft) of the local NAA.
Secondly, in the last two years, EEC/EASA have been making it much harder, particularly if the aircraft are "executive jets" or similar.
In the US (in broad terms) executive transport comes under FAR Part 91, but EEC regards the same operations as "public transport", roughly as per FAR Part 135.
Tootle pip!!