Why is a TV screen a output device?

Both, depending upon it use. An analog camera (like an old Polaroid) is strictly an output device only. However the new digital cameras can act as motion detectors and other types of security related devices with the ability to programmaticly interact with its environment - in this regard the camera is both an input device as well as an output device.

Added:

Additionally, any camera with a digital format can act effectively as an input device, if it connects to the computer via USB. (You can upload pictures directly to the PC, thus input)