It seems to me that the penalty is given for an offence which was not envisaged in the rules.
'Gaining an advantage' has been taken to mean, and I think was intended to mean, that the driver shortening the circuit comes back in front of a competitor. Hence the understanding that ceding the place gained makes everything OK. That was what McLaren (and apparently Charlie Whiting) took it to mean, it seems.
However, we now have a different definition of 'gaining an advantage' which is, in my recall, new. That the returning driver must nullify any momentum gained by their off-track maneuvre.
Whereas it is pretty clear whether or not a driver has ceded a place, how much momentum he has gained is not (in fact McLaren are basing their appeal on the fact that Hamilton was going more slowly than Raikkonen at the finish line speed trap, and therefore had not gained momentum.) Or is it the fact that he did not lose momentum, which he would have had there been a gravel trap?
It is, in my opinion, a good rule but a fairly silly interpretation. It's racing, after all, which is a grown-up sport for big boys and girls going as fast as they can.
The decision is not open to appeal, as far as I can interpret the rules. The regulation of a high-tech modern sport by a bunch of middle aged men in blazers should be. Can you imagine FIFA allowing local organisations to interpret rules as they see fit? I think not...