The military is governed according to the Uniform Code of Military Justice (UCMJ). There are several punitive articles in the UCMJ that aren't crimes at all in the civilian world, such as Article 88: Contempt Towards Officials.

Any commissioned officer who uses contemptuous words against the President, the Vice President, Congress, the Secretary of Defense, the Secretary of a military department, the Secretary of Transportation, or the Governor or legislature of any State, Territory, Commonwealth, or possession in which he is on duty or present shall be punished as a court-martial may direct.

Such a law would violate the First Amendment in the civilian world, but it is necessary in the military world. Here in the US, our military is ran by the civilian officials. Any public criticism of those officials would be seens as undermining that leadership. Sometimes, we had to bite our tongues until they bled but that comes with the uniform. Without exception, everyone I met in uniform understood this.