Fair point, but I think the rules are if you leave the track and gain an advantage then you're open for punishment.
Lewis left the track and gained around 50 yards. Max done the same and gained next to nothing. The gap was pretty much the same as they exited turn 3 if I remember right. Even when the Race Engineer said to "give the place back", they said that it was the wrong call as he hadn't actually gained an advantage and Vettel wasn't attempting an overtake into turn one when Max made the mistake.
Yes, Lewis was in first and ended up first, but at the same time Max was third and came back on third?
I just don't see how you can give one person a penalty and not the other for making the same mistake. I've not watched the footage again, but if I remember it right Max clearly slowed as he went along the grass and ended up level with Vettel, but Lewis just lifted and ended up a fair distance ahead and (as far as I can tell) only slowed when the SC was deployed.