I'm fairly certain that they lend to everyone, not just other tribes. The article makes it sound like the tribes out in the boonies need to do something to survive, and the US federal aid mandated in our treaties is hideously insufficient for their needs. Like, I think that what they're doing is bad because you shouldn't harm people, and you especially shouldn't harm people who are struggling. However, I can at least understand why they're doing it. I can also see that the society they're targeting shares responsibility for the situation. If the US had been continuously honoring the letter and spirit of the treaties we signed with the tribes, then the tribes would've have to resort to exploitative shit like casinos and predatory lending to make money. People will do whatever it takes to improve their situation, and it's a shame that the US has failed in this regard.
As an aside and for the sake of clarity, I think casinos are predatory and exploit people who don't understand their odds, but I'm not against their existence in general, or their existence on reservations. Native American folks can do what they want, it's their goddamned land, and the consenting adults who go to casinos have a responsibility to themselves. I only bring up casinos because of my perception of them and their presence on reservations.
(Also, fuck non-tribe people that use reservations as a way to skirt US regulations. I had no idea that non-tribe people used tribes as fronts for shitty lending operations. Again, kinda smells like something that could have been prevented with proper federal aid.)