While IANAL, my understanding is that the laws under which he was detained were intended for stopping actual terrorist threats; espionage - even if that's what we want to call this - is not terrorism, and I don't think anyone has asserted that Miranda has any involvement with any such plots.

I agree that the "they're just doing this to hurt me" spin Greenwald put forward is likely inaccurate - if Miranda was indeed moving documents (or it looked sufficiently like that from the outside) then claims that the authorities are dragging him into this to hurt Greenwald are flat out wrong - but that isn't the same thing as the detention being "entirely legitimate."