Park Police have been around for a long time as you say, but they were always a separate entity from your basic “park ranger”. Now, they are all vested with police power and I don’t believe that was the case until the patriot act.
I don’t know what else that Communist Act added — however, park rangers have long been law-enforcement commissioned. They were in a separate category from the generalist park ranger (e.g. interpreter, fee collector), but they’ve long had police powers. They were proud to take over the title of “park ranger” (so that no one else working for the NPS was “really” a “park ranger” anymore, and instead, almost treated like a second-class citizen). The general public was too asleep to notice.