Compare it to two drivers braking in to a chicane. The car on the inside going in pushes the other car wide, and that car cuts the second part and takes the position. Nobody would question the need to give the place back in that situation, and the rules show that it's not "cutting a corner", but "outside the track limits" that matters.
Raikkonen took several positions at the 2009 Belgian Grand Prix by going off the course - without being pushed off by anybody else. Sutil also overtook Badoer at the same grand prix by going off the track. I'm not saying that's right, but the precedent certainly seems to be there. It'd be difficult to justify that Hamilton gained grip, traction etc. by being forced off the road.