Lead Balloon, you also need to consider that sentences handed down by the judiciary are not only meant to act as an appropriate punishment for the crime that has been committed, but to also act as a deterrent to others who might be considering doing the same thing.
Now, I'm well aware that the deterrence angle doesn't necessarily always work, after all people still try to smuggle drugs out of Indonesia, people still commit murder in the death penalty states in the US..
The fact is though that while the actions of the individual responsible for those radio calls may not have put lives at risk on this occasion, the possibility is certainly there. In my opinion, passing fake ATC instructions that are acted upon by the crew is as close as you can get to hijacking an aircraft without actually getting on board.
Whether the person charged is guilty or not I have no idea, but if he is he deserves to feel the full brunt of the law. He wont do it again, and hopefully others will have second thoughts as well.