Answering the question posed requires some explanation but it’s important to note the tribe managed to satisfy two key criteria to get sports betting in on its own.
Firstly, state law in NM doesn’t explicitly forbid sports betting.
Secondly, for sports betting to be approved it needs to be absent from a list of prohibited Class III games in the tribal-state compact* (an agreement which aims to keep an equilibrium between two political entities) which it is. Confusingly though it’s not on the list of authorized Class III games either.
The ambiguous nature of sports betting in NM and where it sits within the laws, has culminated in the gray area being taken full advantage of.
It’s a loophole which has helped create the most unlikely of sports betting stories. This said, New Mexico aren’t alone in the emergence of tribal sports betting - similar instances exist in both Nevada and Mississippi.
New Mexico is unique in having a single tribe having a standalone industry though with sports betting active across the whole of the aforementioned states.