In BA Flight Crew orders, there used to be a long list of countries where photography was banned overhead or on airport territory, and the crew were instructed to prevent photography. A lot of countries still do take it seriously, so whilst you can happily snap away, in some surprising countries, as well as eastern Europe/former Soviet states/Asian countries, you may well feel a hand on your shoulder and handcuffs slipping on, and telling people to get knotted won't work! Just be careful and be discrete.