That is no bug – it is a feature :-). If you change your exposure, shutter speed (or aperture or ISO) have to change otherwise it would be complete useless, because the combination of shutter speed, aperture and ISO *is* your exposure. 

**For your 70D** (I tested on my 70D)**:** in manual mode you can´t change the exposure compensation. When you change the shutter speed the exposure *indicator* is changing.