I am an avid bird watcher and while watching birds in a public garden under renovation on the Mithi river that passes through Mumbai, private security guards rushed out to tell me that photography was not allowed. When questioned they said that they were doing as told, apologized and left. In each case there was no official warning which said photography was prohibited.
The reason for preventing photography may differ for each of these situations. In the first concern, in the second a misplaced notion of security and in the third, fear that I might be a press reporter reporting on how public funds were used or misused.