But then, when you are allowed to usurp power assigned to the Legislative branch, what's to stop you from usurping the power assigned to the Judicial?
I'm not getting your point. Suppose Private Citizen Joe Blow sues private company XYZ Corp. for violating the Investment Company Act of 1944. The Supreme Court hears an appeal in that case; how is the DOJ "placing themselves in the stead of the Court" if they don't file a brief?