^You only have to leave a cars width if any portion of the car behind is alongside you whilst on a straight. It does not apply to any stage of the corner itself, and the rules that actually govern what is and isn't allowed during the corner are unwritten (and therefore frequently applied incorrectly, by fans and the stewards).
I've said it elsewhere on this board before, but this incident was very borderline in my opinion. Had Hamilton been ahead like in Japan then there would be no question that the move was legit. However, in this case the cars were roughly level, and if anything having watched that spectator footage Rosberg was actually ahead. Furthermore, this was not just a squeeze to the outside, Hamilton quite blatantly drove into Rosberg to force him wide.