Sony Pictures, is an American subsidiary of Sony. The headquarters and employees are all based in Los Angeles. These are American employees whose emails and social security numbers were leaked, American employees who received death threats, American movie theaters that were threatened with a "9/11 style" attack, and an American produced movie that has been suppressed. While perhaps the Japanese government should be involved as well, I think it's quite obvious why the US government would be involved.
From everything I've read, yes, that is correct. Only Sony Pictures has been targeted, only their emails leaked, only those employees threatened, etc.
Also, you have to understand that Sony is a 140,000 employee organization, with many different operating units that are functionally probably pretty separate. The Sony Pictures network and email system was likely entirely separate from other Sony divisions, especially since one was based in Los Angeles, and others are probably based in Tokyo.
