It's consistency. It's what we've all wanted for the last however many years across all of the rules. In instances like these it's not right, indeed it's far from right, but as long as it's consistent it's as fair as it can be. Show me a case where a driver was punished while backing off, no matter how brief, in any session. I'll bet you'll be looking back a long way.
It's been that way for at least a decade. Like I said, Alonso, among others, did so in 2006 and they weren't even investigated. There's been many more instances over the years before and I'm sure after, as I said, all the way back to ~1997 (feel free to mention any instances I've forgotten since where drivers were punished for such offences).
Was it double waved yellows ("prepare to stop") or just a waved yellow? All I saw were yellow boards. I'll watch Rosberg's lap again tomorrow and keep an eye out, but I didn't notice double-waved yellows when I watched it afterwards.
As I said, Hakkinen drove through double-yellows while waving at the marshals as a way of acknowledging their presence. Nobody has ever been "prepared to stop" in these instances. Ever. Not once. What Rosberg did was clever in that he backed off before braking into turn 8, meaning he lost a fraction of the time he would have done had he done under acceleration or even through the apex. Don't kid yourself that Hamilton wouldn't have done exactly the same if the roles were reversed.
That said we need a Le Mans Series yellow zone. The FIA's argument that the drivers would push to the limit and then potentially lose control entering the zone is a silly one. It would be quite easy to regulate power through the mandated ECU's they already have, and if at the end of a long straight then just introduce it so that the zone starts before the straight. F1 just doesn't want to lose face. If F1 hasn't invented something then it's going to take a heck of a lot of persuasion to get them to implement it... and if a driver dying apparently wasn't enough reason to persuade the decision makers from implementing it then it's not going to get implemented.
I said it more than most after Bianchi's accident - take the situation out of the driver's hands, as if you don't, every single action they take will be in their own interests. Today was no different.