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Yeah, the omission of the Navy/Marine Corps from the act is slightly weird, given that the Marines have the equivalent of a couple of rapidly-deployable infantry divisions; as you note, the Act's provisions are applied to the Navy through regulations. The Coast Guard are a law enforcement organisation as well as a uniformed service, but they're presumably constrained in a similar fashion (and, purely pragmatically, are less likely to have the forces suitable for staging a coup).
Thrustvector-
the Marines have the equivalent of a couple of rapidly-deployable infantry divisions
For those who might be unaware, the US Dept. Of the Navy maintains 10 aircraft carrier strike groups (ships and submarines centred around an aircraft carrier) and nine expeditionary strike groups (also ships and submarines but centred around a smaller aircraft carrier and a batallion of Marines with tanks and vehicles for amphibious assault). A technique used in geopolitics to identify where the US’ truly wants to project its power is to keep an eye on the whereabouts of the carrier groups and the Marine expeditionary groups. The South China Sea seems to be a popular place to go.
Edit- obviously these groups keep their whereabouts hush hush, so when they actually say where they are it’s because they intend to make a point.
Adhiero
Its text only specifies the Army and Air Force. The Navy, its Marine Corps, the Coastguard, Homeland, Prisons Service, FBI, BATF, DEA and Space Force are all federal agencies (edit- not all are technically ‘agencies’) with military or paramilitary capabilities, and aren’t subject to the PCA. They have their own regulations though.