I would not overstate the certainty of the illegality. I've heard many people state this like it is a fact. It is much grayer legal territory. There is a substantial ways to go to determine if it would be found illegal.
The average american commits illegal acts daily, but I doubt you were referring to that.
I'm curious as to why you think so - the fact that he improperly removed and published information belonging to the NSA isn't disputed by anyone. I don't have the title and chapter on hand, but I'm just nearly certain that this is about as cut-and-dry illegal as it comes.
If you can give me a hint as to why it might not have been illegal, I would love to be enlightened.
The average american commits illegal acts daily, but I doubt you were referring to that.