Things you can't do in F1 in 2016:
-Attack
-Defend
-Exceed track limits
-Tell your driver how to operate the car
Things you can do in F1 in 2016:
-Exceed track limits
-Tell your driver how to operate the car
-Stay on the outside of your opponent to get him a penalty
But seriously, I'm not even complaining that much about this specific penalty for Rosberg. It just is the inconsistency of the penalties. Nico's move was a classic dive bomb. You will be first at the corner, but you risk missing the apex and your opponent can overtake you back on the inside. But I guess dive bombs in general are now outlawed in F1. At least for some drivers. Dangerously moving under braking long before the apex of a corner is no problem though, as shown by Verstappen last race on Raikonnen and this race during the incident with Rosberg.
If you are seriously trying to prevent drivers to force another driver off the track, then this was no different than the classic Hamilton move of running his opponents wide on the exit of a corner. But no, that move gets you an A+ for effort because you are turning your steering wheel. Waiting for having slowed down enough to turn in gets you a penalty. Penalize both or neither of it, if the goal is to enforce the rules.
I guess in the future everyone will just stay on the outside of Rosberg, no need to try anything fancy like moving to the inside.
EDIT: This analysis of the incident pretty much covers how I saw it:
https://streamable.com/mh8i