edi local, the fact is there are no hard and fast rules. Some do and some don't. It can be because of local law or bye-laws, military or other security sensitive reasons. Sometimes it can be due to the ignorance of airport personnel and even police! You can happily photograph all day at an airport with multiple patrols passing by and waving at you one day, and being asked to stop and move on another day simply because of which watch is on duty on a particular day. It is quite evident to most that some countries will obviously be more sensitive than others and enthusiasts should factor this in. Unfortunately (for them) some don't.
On the subject of ramp photography, some years back, arriving on a remote stand at BHX with my young grandson, I took a quick shot of him as he came down the steps and got yelled at by a very officious female. I apologised and moved on. I had my photo anyway!