In the days when I used to race single-seaters, one of the circuits we'd visit was Pembrey, in Wales. It's an interesting little circuit, about a mile and a half long, with a couple of reasonable quick corners.
One of these corners, Dibeni, would always be mentioned in the drivers' briefings. Because it was a long, double-apex corner, with a lot of tarmac on the inside, there was a tendency for drivers to shorten the corner by running their inside wheels across the white line.
The Clerk of the Course made it quite clear that if we were seen doing it, we would be shown a warning flag, and if we did it again afterwards, the black flag (Go direct to Jail, do not pass Go etc).
Note that this had nothing to do with passing another car. Just gaining an advantage.
So why don't the stewards at a GP enforce the "cars must remain on track" consistently?
Which would mean, looking at yesterday's race, that anyone leaving the track at Turn 3 (eg Rosberg, Webber) or on the exit of Turn 5 - or maybe 6 (Kimi, Vettel, probably just about everybody) should have been served with a drive through. I certainly don't think that Grosjean gained more of an advantage by leaving the track than Vettel did by repeatedly pushing the limits of the track.
F1 drivers are greedy. If they believe that it's possible to get away with something, they will do it. Firm and consistent enforcement of the rules is the clearest approach.