Lets just say for a min they did rob him of this "good"; Then why did they ignore the assault with a deadly weapon and kidnapping charges? Why were they only punished with some community service? This case reeks of the court wanting press and precedence for future cases like this.
If they did not take his good, then it would be an assault and possible kidnapping but it would be inappropriate to assume the charge wasn't applied as the prosecution found a conviction.
The problem here is more or less a double jeopardy issue to why they seemed to ignore it I guess. The actual conviction contained an element of unlawfully in which the assault would have been covered (how did they unlawfully take the good and appropriate it to someone else? By assaulting the kid and holding him at knife point). To charge them for the assault (and possible kidnapping) on top of the convicted charge would have the effect of convicting them twice for the same offense. Once for the offense itself, and once for the offense itself combined with other actions that made it the charge they where actually convicted of. So the assault was actually addressed, it just didn't come right out and say robbery by assault or whatever to make it obvious. Had the taking of his "good" not existed, it would have likely been more clear. Had the robbery part been an enhancement to the assault law (for instance, robbery in the US usually gets one penalty and the use of a firearm to commit the robbery causes a more strict penalty to be applied) it might be more obvious.
An example of this might be murder where someone was beaten to death. The act definitely would have contained an assault element, but because it would be the means to which the murder took place, you don't see a separate charge for it in the conviction. There may have been an actual assault charge as part of the case before the court but the higher offense would negate it if convicted on it.
As to why they only got community service, that is a guessing game as I do not see any explaination for it anywhere. However, I have noticed that in Europe, there seems to be a mentality of being soft on crime with the focus of punishment being rehabilitation rather then actual punishment as it seems to be in US cultures. Also, age seems to have played an issue here. The kids were barely teens so while they would be old enough to know right from wrong, they may not have fully understood the consequences of their actions in the way a full fledged adult might. I've seen studies that say your frontal cortex, the part of the brain that associates consequences with actions, doesn't stop developing until the age of 22 or so which seems to support that (as well as explain why arbitrary ages are assigned to when someone is an adult).
Knowing this, and the ages of the participants, I would guess to say it played a large role into why they received community service instead of prison terms. But it is not exactly a slap on the wrists either. Supposing they pulled an average of 3 hours a day of non-paid community service 5 days a week, they will be dealing with this for 2 years.