Drivers are allowed to defend their position using any part of the track.
What they are not allowed to do is make 2 moves, and in no way shape or from did Rosberg move twice.
He telegraphed the fact he was going to the outside. It is then up to the following driver to use the track to over take.
If he gets penalised, then it sets a dangerous precedent for leading cars not being allowed to defend in an way.
Squeezing cars out onto the kerbs will also be banned in such a motion, and this can never be allowed should F1 want to be seen as a series that allows overtaking.