Sounds like it’s surveilling the reservation to me. The stated purpose is to monitor border crossings, but that’s not the same as surveillance of the border.
https://en.wikipedia.org/wiki/Tribal_sovereignty_in_the_Unit...
As someone living in Tucson, and having to go through these Border Patrol checkpoints, it does feel like they've gotten more aggressive over time. The last time I went through a checkpoint, they didn't even ask about citizenship, just wanted to run their dog over the car sniffing for drugs, which I feel is a 4th amendment violation, even though the supreme court probably disagrees.
I'm not sure if they have any legal recourse, but I'd love to hear a lawyer chime in. IANAL.
per se, latin for "in itself"