Eh, what? What monopolies are granted by law on the basis of natural monopoly theory? As you state, there is no reason to put them into law; natural monopolies

Eh, what? What monopolies are granted by law on the basis of natural monopoly theory? As you state, there is no reason to put them into law; natural monopolies are usually the ones who are broken up by regulators, to enforce competition (e.g. telecoms, energy manufacturing/distribution, public transport, ...)