The details of plenty of public matters are decided by regulation under relevant government legislation: if it were not so, governments would grind to a halt. In other cases, matters are worked out by local government decree. (In the English legal system, and I imagine most others, local governments don't legislate, but they decide on plenty of matters, such as local planning, whether a restaurant can put tables and chairs on the pavement outside the premises, and so on and so on.)

So I don't think that this situation in the NT is particularly unusual, it is just the particular rules that seem bizarre (to outsiders, at least).