If you are asking specifically whether photographing them is legal, In the US, you have the right to photograph anyone in public with a drone or anything else, same as anyone else, unless they have a reasonable expectation of privacy, (such as their backyard or rooftop etc.)
In the US if it is not illegal then it's legal. Not trying to be smart a$$ but many will simply not do something if in doubt, so eliminate the doubt. Find out if there is any restrictions on launch or landing in the area, find out if you are closer than five miles from an airport with a control tower, and notify them, and don't fly directly over or dangerously close to anyone. If you've covered your bases, you are legal, and don't let anyone tell you otherwise.