It would make sense. If there were some kind of agreement in place between part of the government to exchange technology for humans, then they could really only do so on land owned by the government. A National Park is the perfect place because people show up there willingly, and often times there is a great distance between people, so the aliens could capture their prey without being seen, and keep the whole thing going while unseen.
Agreement? National parks are totally controlled and run by the Fed. It’s built in.
And yeah you are right. It’s a perfect setup, muh we don’t know, lots of land, people go missing logic is done stable by the hordes of npc’s