From my own experience the vast majority of 'N' reg owners act responsibly and keep their aircraft and their own competence as they should. Cowboys have been around for decades and to them it matters little whether the aeroplane and/or the pilot has FAA oversight or EASA or the UK CAA. It is not possible to comment on the Sala case.
Murder, robbery and fraud have been rife for thousands of years and so has the law against them but ................ Criminals and cheats always find a way to bend and break the rules and always will. Nation states, subject to ICAO, find it almost impossible to ban or restrict foreign registered aeroplanes and crews in the way you suggest Mike. EASA tried to limit U.S. aeroplanes, pilots and engineers some years ago and they fell flat on their face, under FAA pressure. In many ways this attempt by EASA's two left feet simply opened the doors.