Completely legal. See for example: link

Tribal and native american preference laws are an exception to the normal Equal Opportunity laws and are most often guaranteed by treaties between the various tribes and United States of America. (Something to do with the fact that the Indians were residents of this land long before the British and French were).

Not only is it legal for them to advertise in this way, they have a ready-made defense – their treaties – if they are ever sued.

I guess that makes sense.

It makes sense because it is on tribal land.
Connecticut Mashantucket tribe had their casino and other buildings on the reservation built by Native Americans almost exclusively, that was in the early 1990’s.
They also had requirements on building materials like minimum of “50 year shingles” for roofs.

Looks like it is legal from the answers already written above. It doesn’t surprise me. Another group that gets preferential hiring is military vets. I know they do for federal jobs, but I think maybe they do even in the private sector, I’m not sure of the laws around it. Job applications usually ask if you have served in the military, and I think that’s why.