There's also an element of what it actually gains them. Vettel exceeding track limits might gain him what, 0.1s a lap? To penalise him (or any other driver who does it either on purpose or by accident) 5s just wouldn't work. I'd say they've got it fairly well set out anyway. They know the rules, they're being watched and if they look to be regularly taking the piss they're penalised. Job done. Also, to be fair, although I really dislike Vettel, if I was in anything other than a Mercedes I'd be looking to exploit every possible advantage as well!