I believe it was the Patriot Act that did make them park police, instead of just rangers.
The Park Police have been around a very long time too — at least since the late 70s/early 1980s in some of the urban parks. Regular NPS park rangers started going to “cop shops” and getting their law enforcement commissions back then too. Unfortunately, too many of them were wanna-be cops who could never make it in the big city. And, their attitudes have become more and more hateful-toward-park-visitors. Don’t get me started on my stories.