The answer to your question is quite simple. This is apparently implemented as a security feature, which is probably less security related and related to the sales problem. Adobe loves to block certain features intentionally in order to create the opportunity or need another product, as well as greater revenue.
I tried to study this for you to see if there is some kind of dirty hack where you can attach a camera or something else and override the binary data sent to the stream as you can with Audio, but unfortunately as far as I know, such hacking is possible. More nfo here: NetStream.appendBytes
Update
Perhaps you can do something hacky using ManyCam, which is a virtual webcam driver (from what I understand). This will provide a valid camera that you can select from the flash, and you can also select the video file as the source file for ManyCam. See http://manycam.com/user_guide/#HowtoSelectaVideofileasthePictureSourceforManyCam
Update # 2
If you are looking for something open that will do the same as multi-camera, check the following:
http://code.google.com/p/webcamstudio/wiki/VideoSourceMovie (GPL)
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