Exposure is defined as the total quantity of light that hits the film or sensor during the time the shutter is open.
Exposure compensation in Tv or Av modes will change the shutter speed or aperture, which in turn changes the total amount of light that hits the sensor, i.e. it changes the exposure.
When shooting in manual mode the aperture and shutter speed are chosen by you, and the camera is not allowed to change them, hence there's nothing for the exposure compensation to do.
Canon is both correct in no offering exposure compensation in manual mode, and massively short sighted in not offering an "ISO compensation" feature which biases the chosen ISO value up or down when shooting in auto ISO mode.