Welcome to Ten Tenths, Andy.
One thing we ought to take into account with the stewards' inconsistency is a comparison with football. During the World Cup, people were given cards when nothing wrong had been done and not given them when something quite blatantly had. So at least F1 is not alone on that score.
My take on Massa's incident was that he seemed to over-react to the Mercedes (although it did look like it might eventually hit him). By moving to the grass the way he did, maybe he guaranteed an accident, which might not have been the case had he stayed on track.
I can understand penalising Petrov more because he just moved left and made quite an error of misjudgement by moving the way he did so soon when it turned out there was a car within range behind him. I don't buy his account about having to move left because of a car doing likewise to his right. I watched replays and can't see anything. On the other hand, I'm not a big fan of penalising drivers for errors. It's an odd precedent and quite new within F1. It goes under the "causing an avoidable incident" rule I think, but what incident isn't avoidable if we're going to be pedantic? We want drivers to go for moves.
This sort of penalty was at its worst in 2008 in Fuji when Hamilton got one for out-braking himself and going off WITHOUT even having hit anyone, and Bourdais got a ridiculous one for colliding with Massa when Massa turned in on him. Anyway, there were a lot of judgements not going McLaren's way that year (Belgium for instance). So maybe I shouldn't compare it to the nadir of stewarding that was 2008.
Petrov went too early with that move as drastic as he made it, but again, it's an error and they all lose out due to unfortunate circumstances at times. It's not a reason to penalise someone.