When I'm in a public space, outside or inside, and the use of a camera is not an issue, I freely take photographs of everything around me, if I find it interesting. That doesn't require permission from anyone who is captured in a photograph unless I publish that person's image in print, which I don't do. Photographs are a valuable tool for artists, no doubt - to catch action, light, color and figures - but I suggest it is not taken literally when creating a painting or drawing from that photograph. That's just boring to me. Part of the learning process is using references as a way of being somewhat accurate, but giving the scene more of your own interpretation.
I do enjoy painting people in museums, as you can see - and there are policies in each and every museum which I am totally aware of before I step through the door. Most privately-run museums forbid any use of cameras, obviously because the artworks are privately owned or on loan. A lot of special exhibitions are off limits to photography for the same reasons. If you're interested in using your camera inside of any space, I'd suggest you pay close attention to the rules and always be as non-invasive as possible. Simple as that.