It's a military airbase, with military weapons on it. Was anyone actually expecting Open Day?
You wouldn't jump off the bridge because it would be a stupid thing to do, same rule applies here.
Edit : Just wanted to add, the whole reason the airbase is probably being included in the game is because the relationship between Greece and nearby Turkey is still volatile, and should Turkey ever choose to make an aggressive move against Greece or vice-versa, Lemnos would play a prime role, and I wouldn't be the least bit surprised to find that is one of the very scenarios Arma III is considering. So we aren't just talking about taking photos of an Airbase, we are talking about taking photos, without permission, of an airbase in a volatile area that could be targetted if the delicate relations between Turkey and Greece were to break down again.