27.7 requires the leading driver to leave room, if there is a significant portion of the car attempting to pass alongside.
Car 44 had a portion of his front wing inside car 6 small fractions of a second prior to car 44 having to leave the right side of the track to avoid an initial collision, which may have led him to believe he had the right to space on the right. Once on the grass on the side of the track car 44 was no longer in control of the situation.
Having heard extensively from both drivers and from the team, the stewards determined that car 6 had the right to make the manoeuvre that he did and that car 44s attempt to overtake was reasonable, and that the convergence of events led neither driver to be wholly or predominantly at fault, and therefore take no further action.
http://www.f1fanatic.co.uk/2016/05/15/hamilton-rosberg-stewards/