In the usual drafting scenario, the chasing driver will pull out of the stream to attempt an overtake, and then the defending driver is allowed that single change of line in defence of his position; but here Hamilton moved first, and Petrov followed
If He was close enough to overtake, why not just stay on line and drive past? There was more than enough space. It's not like Lewis would have driven in to Him if He'd pulled along side. It was Vitaly's decision to follow in Lewis's wheel tracks and stay in His mirrors. I wouldn't be inclined to really call it dangerous since it was well before the braking zone.
At worst it was a little cheeky, but fair enough for the stewards to issue the warning.