To whom and why?
A Reserve, is by statute, a Crown reservation, not subject to the applicable common law of the land. As seen on other Crown Reservations, such as Military bases. Which are held to Military Law, first. Unless otherwise legislated. As is with Treaty lands/Rights and Reservations.
Secondly, the majority of Native, Crown Reservations, are not incorporated, not ceded to the Crown, sovereign territory. Enacted and enforced by Treaty.
Those who live and work on said Crown Reservations, are not subject to taxation by the Crown, as laid out in specific and varying Treaties. Not all Native Reservations, nor Native peoples enjoy this Treaty right. As they are held to their specific Treaty, regarding how they are subject to or viewed as, by the Crown.
Furthermore, in several cases, Native Bands have ceded sovereignty to the Crown, in the form of incorporation, or ceded territory for financial profit. But this is an entirely different topic altogether, and the basis of much turmoil amongst the different Nations and Bands themselves.