Planespotting is an occupation pretty much limited to Britain and north-western Europe. Mad dogs, Englishmen and the odd Belgian sit around airports (that's around, usually in a field, not in a cosy terminal) and watch planes like kids used to sit on bridges and watch trains a hundred years ago. Which is harmless enough when you're watching British Airways 423 arriving at Heathrow from Amsterdam. Which is tolerated and even accommodated at air bases in the UK and probably elsewhere too. The nutjobs who sit in cold fields in the morning armed with thermos flasks, pens and notebooks, and expose themselves to the noise level of air traffic at close quarters gather information on aircraft types and designations.
Now picture this: you take your binoculars, cameras and notebooks to a country that has spent the best part of the last 180 years at war or pretty damned close to it, even as recently as 1996. Coincidentally, it's also a country where planespotting as a hobby is viewed with incredulity
("they WHAT?"). You hang around an airfield and take notes about military aircraft and their identification. Notes which could perceivably be of value to a certain aggressive neighbour six times the size of said country (whether they actually are is irrelevant from a legal standpoint). Information which the organisation in question here would not gladly volunteer and hand out to whomever asks for it (the fact that some data are available on the internet is also beside the point--they do not appreciate the fact that they are). You pride yourself on the fact that the information you gather is sometimes preferred by officials over official data. Now blame them for thinking you're a spy and not some loony who likes to collect plane sightings for his personal enjoyment and perusal. Spies haven't worn dark glasses and trenchcoats since they last sneaked around East Berlin, they look like tourists.
Planespotting is not a good idea where it's unknown and unwelcome. Greece is an example of a bad place to be doing it but you would probably not be any better off as a German tourist snooping around Andrews AFB with a pair of binoculars and a notebook any time after September 11th, 2001. As with every obscure pastime, it's liable to be misunderstood and you take your chances if you're actually dumb enough to try it where it's not appreciated. If you can't grasp this, birdwatching is probably a safer, more suitable (and much more tranquil) occupation. And next time you see someone with a camera skulking in the bushes around a military base in Northern Ireland, hand them a cuppa and wish them well. They're just hobbyists.
I don't think I would have ever heard of this peculiar hobby had the following event not been in the news in 2001:
A number of British and Dutch planespotters were arrested near a Greek air force base and charged with espionage. They were later freed on bail pending reduced charges (and all but the one who didn't show up in court were later acquitted on appeal). My personal view is that they're overreacting--after all, a lot of military information, probably including what they collected, is exchanged through official channels. On the other hand, having lived there for a long time, I understand just how touchy the subject of random foreigners gawking at their operational military hardware without being asked to is. You wouldn't do it, for example, in Israel, don't do it anywhere else where there are real geopolitical and national security issues that you don't fully grok. Planespotting is a safe hobby if conducted with common sense.