Snowden made public information that may be used to track terrorists by examining metadata. If they know this, they can change the pattern by which they communicate. Snowden's leak allows them to more confidently secure their communications against monitoring by the NSA. That's the logic for the argument that he helped Al Qaeda.
This ties in very interestingly to Manning's case, and the verdict is, to me, a bit contradictory and I'm not sure what to make of it.
The ruling found Manning innocent of aiding the enemy. But at the same time, he was found guilty under the Espionage Act, therefore making leaking to the press an illegal act.
Additionally, this fascinating tid-bit was included, which would apply to this discussion.
By that fact, I suppose, it seems Snowden would not be happy with the verdict.Manning was also found guilty of "wrongfully and wantonly" causing to be published on the internet intelligence belonging to the US, "having knowledge that intelligence published on the internet is accesible to the enemy". That guilty ruling could still have widest ramifications for news organisations working on investigations relating to US national security.