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)