If anyone's interested, I found the appeals court ruling finding him not guilty of violating federal law, and leading to his release, on PACER, uploaded to the Internet Archive so you can access it without a paywall via RECAP:

https://ia700209.us.archive.org/9/items/gov.uscourts.nysd.35...

The basic argument hinges on the three counts, related to three laws he was alleged to have broken: the Economic Espionage Act, the National Stolen Property Act, and the Computer Fraud and Abuse act. The third charge was dismissed by the district court because it rested on the fact that he had either accessed systems he was not authorized to or exceeded authorized access. However, he was authorized to access the source code in question, and what he did with it afterwards has no bearing on whether he exceeded his authorization, so it doesn't fall under the CFAA.

The district court did convict him on the first two counts, but the Appeals Court reversed. Their argument is that the National Stolen Property Act doesn't apply because it applies only to actual physical goods, not mere intangible ideas. Had he photocopied the source and walked out with it, or loaded it onto a thumb drive at the office and taken that with him, it would have counted as stealing a physical good, but merely uploading it to a server and downloading it onto a thumb drive later does not count.

The court further argues that he did not violate the Economic Espionage Act because the clause in question he was prosecuted under specifically requires that the "trade secret ... is related to or included in a product that is produced for or placed in interstate or foreign commerce". Since Goldman Sachs' HFT trading system entirely proprietary and internal and not produced for or placed in interstate commerce, that particular law does not apply. Apparently Congress specifically intended this restriction, because earlier drafts of the statute had broader language that merely included "proprietary economic information having a value of not less than $100,000". The fact that Goldman Sachs uses the product for interstate commerce is not compelling, it had to itself be produced for or placed in interstate commerce.

That last part is interesting. It implies that if you run proprietary, internal code that is not sold or intended to be sold in the future, you appear to lose federal criminal trade secret protections. It's interesting that they tried to prosecute him on theft, trade secret infringement, and exceeding authorized access, but not copyright infringement. From my reading even unpublished work is subject to copyright.

Neither the original conviction nor the appeals court opinion ever addressed the copyright issue. In order for him to have stolen something, it would have had to be something of value; so why wasn't he further prosecuted for copyright violation? From the documents I read (not all are available on PACER), the copyright question never even came up.

More documents from the case:

Motion to dismiss the original case in district court: https://ia600209.us.archive.org/9/items/gov.uscourts.nysd.35...

Government's response to the motion:
http://www.archive.org/download/gov.uscourts.nysd.358303/gov...

Affadavit of the investigating officer:
http://www.archive.org/download/gov.uscourts.nysd.358303/gov...

District court's opinion dismissing the third count but refusing to dismiss the first two:
https://ia600209.us.archive.org/9/items/gov.uscourts.nysd.35...

List of files requested in discovery, to demonstrate that what he took was insubstantial and not proprietary:
http://www.archive.org/download/gov.uscourts.nysd.358303/gov...

A few of the things he had downloaded were their version of the Erlang platform, which is available under the Erlang Public License, a derivative of the Mozilla Public License. So it would be more fruitful to debate the merits of that license, not the GPL.

All of the currently uploaded items in the docket:
https://ia600209.us.archive.org/9/items/gov.uscourts.nysd.35...