why cant you use a canon EF-s lens on a Mark III camera???

Answer 1

They would produce massive vignetting.

Canon dSLR cameras use one of three sensor sizes. The 1Ds Mark ll, the 5D, etc have a full frame-sensor. These sensors are the same size as a frame of 35mm film.
The 1D Mark lll and a few discontinued models have a 1.3 crop factor. These sensors are a bit smaller.
And the 30D, 400D, etc have a 1.6 crop factor. Smaller still.
Canon's EF-S lenses are designed to have an image circle that barely covers the small 1.6 sensors. On 1.3 and full-frame cameras, they will only project an image onto the central part of the sensor. The edges of your pictures would turn out black.
Even so, you should be able to shoot with EF-S lenses on the Mark lll. And with zoom lenses, you'll probably even cover the entire sensor at the tele side of the zoom. Nikon has the same thing going on with their DX lenses. I use a Nikon dSLR myself, and these all have a 1.5 crop factor. With my 17-55mm DX zoom, vignetting only occurs on a film body in the 17-35mm range.