"The Swedes were successful, but one of them was caught on the last mission. He didn’t confess to what he was doing ... even after stringent interrogation ... and Riddle falsely concluded that he was simply trying to escape the country. So, as usual with his paranoia, he had his Death Eaters erect their own Wards around airports, to alert them of Magical Movement in order to prevent future escape attempts. He was convinced that there were European spies everywhere, so he made it illegal to travel abroad without his own hand-signed permission ... which he never gave out once, so far as we know."