The US Govt argument is very simple, and not so controversial.
They are arguing that you don't own files, you own the information in them. They are considered separate things. So Goodwin does indeed own the information contained on the seized Megaupload servers, but he doesn't own the files and doesn't have the right to access them. It's just too bad that Goodwin left the only files that contain the information in a single location.
It actually isn't a bad argument to make when products are so easily and cheaply duplicated because they are digital items.
You can copy and distribute as many copies of a file as you like because it has no ownership. You can have a hard drive full of mp3s, videos, games or eBooks, and share them with anyone else, so long as you don't access the information the file contains by opening it with a program that can access the copyrighted information. So a file of random data has no value in every sense of the word. Neither would a hypothetical file so heavily encrypted that it couldn't be opened. But the information in a file does, so it is the utilisation of the information that would be the illegal offence.
RIAA and other copyright enforcement groups should be thrilled at this approach. Why? Because then it isn't a single offence of having or distributing a file, it a much larger number of offences because the offence occurs every time you view, listen or read the information that the file contains. How many bazillions could they sue an individual for then?
By paying money when you receive the file, you aren't paying for the file, you are paying a licence fee to view the information in the file. A licence that isn't transferable, so by copying the file and sending it to someone else isn't illegal, just if that person then opens the file.
Of course, I would expect the US Govt argument might change if someone passed me a file taken from a CIA server that detailed all the dirty little details of CIA ops over the last 60 years. Then merely possessing such a file would be a rendition-worthy offence, regardless of if I had opened the file or not.