If he did attempt to break the password hash, he conspired to hack military systems. I don't see why someone who attempted to break a password hash for access to classified documents shouldn't be prosecuted, journalist or not.
Presumably there are quite a few reasonable people who disagree on the necessity of prosecuting “crimes” like this. Why’s your way the right way?
Here's a better lens: Assange attempted to solve a math problem.
Solving a hash is not the same as breaking a physical lock, or attempting entry.