Christ. I agree that his comment was a bit harsh, but there really shouldn't be any punishment for this. Hell, I think the max should be a load of community service. But 8 years in a prison with murderers, thieves, rapists, actual terrorists, kidnappers, etc? It makes me furious that this man's life is ruined [to an extent] due to this. When trying to get a job, most folks will see that he was in prison and decline him without looking into the actual crime.
It seems unconstitutional to me. A violation of Amendment 8 - No Cruel or Unusual Punishment (or something of that sort). Also, there is free speech. I know that it isn't right to talk about killing some folks, but he still does have that right.
Unless there was any evidence of previous messages he posted, items in his household/possession, a record of assaults, etc., he should just be slapped and told to be nicer next time.