The US government cannot copyright works that it creates because they are done on behalf of the public. However, it can trademark phrases, artwork and other associated things with government departments to protect both the government and the public from any confusion as to whether or not a private entity is a representative of the government agency.
That is neither here nor there with regards to Goldman Sachs. They are a private company who has received an investment from the government. That doesn't make them a governmental or quasi-governmental agency or department.
With regards to Goldman Sachs' claims, sadly I think Mr. Morgan has made a mistake by filing a lawsuit in court. He should have waited out the harassment until Sachs took him to WIPO. Under WIPO policy he wouldn't have to pay anything to the arbitration panel to defend himself, and he would most likely have a solid claim. By taking it to court, you end up leaving the decision in the hands of a judge - or worse, a jury - who will more likely be ignorant of the policies regarding domain names and is likely to side with the larger corporation.