The "Experimental" aircraft I have flown were given a special flight authority, to permit flight for a very specific purpose, usually testing a new feature or design change, which required flight testing. For this, I must specify the reason for the flight (the "experiment"), and a flight test plan to be followed. It is nearly unheard of that such aircraft cross boarders, other than to attend airshows (Farnborough, for example). The flight permits were both calendar and regionally limited. I can certainly understand why a nation would want to restrict the flight of aircraft which were doing an experiment.
I've also flown amateur built aircraft, which were certainly not "experimental" (I hope!). I know that Americans seem to stick "Experimental" on their amateur built aircraft, I really wonder why. What's the "experiment"? The FAA system, I suppose - apparently not followed elsewhere.