>>realce+f7
In the scenario outlined there would be no unauthorized access to any systems, whether owned by Bitfinex or anyone else, so I really don't see how the CFAA could possibly apply here. As for the cryptocurrency network itself, the protocol is that anyone who has the private key is authorized to spend the corresponding funds—how the key was obtained is irrelevant.
Of course, correctly guessing a 256-bit random private key is exceedingly unlikely, though if they key is based on a lower-entropy password (a "brain wallet") then the odds of a correct guess improve dramatically.