Second, what proof do you have? The SecDef says they didn't.
Well, to be realistic, I'm pretty sure Gates isn't going to come out and say, "Look at these documents right here! These are incredibly sensitive and compromise our combat operations!" His response is pretty pat - when you've been hurt, you don't let your opponent know or he'll just continue to exploit your weak spot. Conversely, when you're not hurt you try and falsely hint at vulnerabilities that aren't there. All part of the game.
And anyway, we didn't execute (actual charged and convicted of various crimes) US military personnel in the 70-80's, when several people passed to military secrets (real secrets) to the USSR.
Many of those people weren't executed because as a condition to avoid the death penalty they agreed to spill their guts about their contacts and handlers - they had info which was useful. Assange doesn't. Although I gotta say that it is a little disturbing to advocate killing the guy - he's just doing what his nature is. If we tried to kill everyone that had ill will toward the U.S., wel, we could spend our way out of recession just on ammunition production.
I could see hanging his source from the rafters, though. I wouldn't shed any tears over that.