Having read this weeks Autosport, there is a small panel explaining the view of the stewards. This that while Hamilton backed off and let Kimi pass, he would not have been as close to Kimi to get by at La Source had he taken the chicane properly. Also, when he let Kimi retake the lead, he ducked straight back into his slipstream, therefore making the overtake easier. Had Hamilton taken the chicane, then he would have still been in the dirty air off the Ferrari, and so would not have been in a position to overtake where he did. Also stated is the fact that Charlie Whiting, on review of the race, concluded that an offence had occurred. The result was the penalty given.
This precident had been set at Suzuka in 2005 when Alonso jumped a chicane (passing Christian Klien). He backed off momentarily before passing Klien again. He was then told to let Klien pass again so as to avoid a penalty.
As a lifelong Ferrari fan (who dislikes McLaren with a passion!) I hope the penalty stands and is not overturned in the court of appeal.