Consider that people may go to national parks to experience the quiet majesty of nature[1], an experience that would be disturbed by a quadcopter hovering a few feet away taking a glorified selfie.

Also consider that there are wild animals nearby who are not used to drones. Areas of Yosemite are closed to climbing annually for raptor nesting, for instance.

[1] Yes I recognize that Yosemite Valley might not be the best example of this...