A flash on the camera's hot shoe is in a known location relative to the film plane. From that, it's easy to calculate where to place the zoom head so the light covers the area the camera actually sees. The extra energy spent lighting the area seen by a 24mm lens when the actual focal length is 85mm would simply be wasted, and that saps the batteries and increases recycle time.

For the remote flashes, the focal length doesn't have any meaning because the only part of the system with any idea where they're placed and how they're being used is the photographer. Speedlights with zoomable heads give you the option of adjusting the focal length manually in remote mode. That can be used to limit where light is thrown if you have enough information on hand to come up with the correct setting.