The excuse that he can't make a case in court is way more nuanced than your linked article indicates. Snowden has been charged under two counts of the Espionage Act (along with theft of government property): Section 793(d) and Section 798(a)(3) [1]. The Freedom of Press Foundation article cites the John Kiriakou, Stephen Kim and Bradley/Chelsea Manning, who were all charged under 793(d), as well as Thomas Drake, who was charged under 793(e). 793(d) reads as follows (emphasis mine)[2]:

(d) Whoever, lawfully having possession of, access to, control over, or being entrusted with any document, writing, code book, signal book, sketch, photograph, photographic negative, blueprint, plan, map, model, instrument, appliance, or note relating to the national defense, or information relating to the national defense which information the possessor has reason to believe could be used to the injury of the United States or to the advantage of any foreign nation, willfully communicates, delivers, transmits or causes to be communicated, delivered, or transmitted or attempts to communicate, deliver, transmit or cause to be communicated, delivered or transmitted the same to any person not entitled to receive it, or willfully retains the same and fails to deliver it on demand to the officer or employee of the United States entitled to receive it

793(e) is similarly worded. It's not that the government doesn't need to show damage - they can't. It's not applicable to the charges. The law is written in favor of the leaker - so long as he or she believed that what they were disclosing would not cause harm to the US, they can't be found guilty under this law. They could reveal nuclear launch codes to the Russians and in the process destroy the entire state of Ohio; so long as they didn't believe that what they were doing would cause harm, they're not guilty under Section 793. Snowden could easily take the stand and say "I didn't believe this would cause any harm. On the contrary, I believe what I leaked was of benefit to the US." The problem for him is that it would probably be hard to find a jury who believed that.

798(a)(3) is ambiguously worded[3] (emphasis mine):

Whoever knowingly and willfully communicates, furnishes, transmits, or otherwise makes available to an unauthorized person, or publishes, or uses in any manner prejudicial to the safety or interest of the United States or for the benefit of any foreign government to the detriment of the United States any classified information...

... (3) concerning the communication intelligence activities of the United States or any foreign government...

I'm not sure how the courts would decide to interpret the statue. Is it "(communicates...|publishes|uses) in any manner..." or "(communicates...|publishes|uses in any manner...)"? In the former case, the US government does indeed need to show damage; in the latter case they don't. Since there's not a whole lot of case law on this, it's tough to say. I'd imagine Snowden's defense (if he ever faces trial) would probably push to have it interpreted the first way. Note also that this law only concerns specific types of classified information, while 793(d) is more broad in what it covers.

It's also worth noting that the whole "Snowden couldn't get a fair trial" meme is being pushed by Glenn Greenwald (the primary beneficiary of the Snowden leaks) and people close to him. In the case of your article, Trevor Timm is a co-founder and board member of the Freedom of Press Foundation; Greenwald is also a founder and board member. And, incidentally, Edward Snowden (who would benefit the most from public perception that he couldn't get a fair trial) is also a board member[4].

[1] http://msnbcmedia.msn.com/i/MSNBC/Sections/NEWS/A_U.S.%20new...

[2] http://www.law.cornell.edu/uscode/text/18/793

[3] http://www.law.cornell.edu/uscode/text/18/798

[4] https://pressfreedomfoundation.org/about/board