It's more than that. New York is telling the Indians to become tax collectors. New York already "charges" tax to non-Indians by requiring residents to declare purchases out of state.
Wrong again. The whole point of the new law is to impose the tax at the distributor level, before it even gets to the reservation. However, even if we assume that the Indians are "collecting" the tax, the Supreme Court has held that the tribes can be required to do that if they wish to sell to non-Indians.