I'm sure people don't pick a spot to sit/stand/walk/etc thinking, "Oh, you know, I might be in someone's shot..." I think if people had that conscious thought, they probably wouldn't do it, but I'm sure most people don't even realize it... especially people who aren't photographers. Visitors to the spot are just trying to enjoy the beauty of the scenery just as much as the next person, and they shouldn't have to be worried about being in the wrong place at the wrong time because they could potentially be disturbing someone's photograph. People just don't think like that, nor is it necessary for them to do so.
I agree. I don't see any reason why people should feel any special obligation to watch out for photographers. Rarely has the tableau been created specifically for photographers (Kodak Hula Show, yes; Grand Canyon, no)--we're usually the ones seeking to use it in a less common manner.