I'll admit this was the first time I'd really considered this at all-- but I went back to Plutarch (the go-to Theseus source for everyone) and took a look at what he had to say in regard to Theseus's kingship, and it sounds like Ms. Hamilton consulted Plutarch for her take on this as well. Plutarch says a couple of things along these lines, crediting Theseus with everything from the formation of Athens itself as a true city to coining currency and instituting the Isthmian Games. But in particular to this issue, Plutarch says:
[...] whereas before they lived dispersed, and were not easy to assemble upon any affair for the common interest. Nay, differences and even wars often occurred between them, which he by his persuasions appeased, going from township to township, and from tribe to tribe. And those of a more private and mean condition readily embracing such good advice, to those of greater power he promised a commonwealth without monarchy, a democracy, or people's government, in which he should only be continued as their commander in war and the protector of their laws, all things else being equally distributed among them;- and by this means brought a part of them over to his proposal.In essence, in order to consolidate the people of Athens and his own power, he cut a deal with the people who opposed his reforms. Good politics, really, and besides which, it freed Theseus to go about taking care of business elsewhere instead of constantly going from community to community putting down disagreements. If Attica were at peace, Theseus could continue hero-ing it up and gallivanting around with Heracles and Pirithous.
But we also have to remember that Plutarch's purpose in writing this was to elevate Rome, first and foremost, by comparing Theseus to Romulus as a ruler, and Plutarch was writing in the First Century CE, and Theseus, if he lived, was born a generation or more before the Trojan War, an event which took place 1200-1300 years earlier.
Plutarch goes on to say this:
He then dissolved all the distinct statehouses, council halls, and magistracies, and built one common state-house and council hall on the site of the present upper town, and gave the name of Athens to the whole state, [...]. Then, as he had promised, he laid down his regal power and proceeded to order a commonwealthTheseus continued to expand the influence of Athens by inviting anyone who wished it to become a citizen of Athens to enlarge the population. Again, this would have resulted in fewer border skirmishes and less infighting around the countryside requiring Theseus's mediation. Plutarch tells us:
Yet he did not suffer his state, by the promiscuous multitude that flowed in, to be turned into confusion and he left without any order or degree, but was the first that divided the Commonwealth into three distinct ranks, the noblemen, the husbandmen, and artificers. To the nobility he committed the care of religion, the choice of magistrates, the teaching and dispensing of the laws, and interpretation and direction in all sacred matters; the whole city being, as it were, reduced to an exact equality, the nobles excelling the rest in honour, the husbandmen in profit, and the artificers in number.From Plutarch's commentary we are left with the impression that above all Theseus wanted order in his kingdom. He wanted peace. Perhaps he did lay down his "regal power" but he did not give up his kingship. He created a system which would keep the disparate peoples and tribes from warring constantly. But why would he be so concerned with expanding Athens and consolidating the people of the lands into one united city if it weren't to bring them all under the umbrella of his influence and control? Theseus was no Solon, who after accomplishing his reforms for Athens, abandoned his power completely and left the country. Theseus stayed on as King.
So I guess to answer the original question, I take Plutarch with a grain of salt in this instance. I expect Theseus did what he had to do in order to grant himself a margin of freedom and some peace of mind. By consolidating Athens and banding the people of Ionia together under his rule, he ensured their safety whether he was physically present or not-- no one outside of Attica would wittingly wage war against a unified force of that size, and his own citizens would not be ripping one another apart when he turned his back on them for more than a week. In other words, with his own people happy and his forces swollen, he could leave without fearing that he'd return to a usurper on his throne from within, or an external king annexing pieces Theseus would then have to wage a war of his own to get back.
Perhaps we should call Theseus the patriarch of good politics rather than democracy. But whatever else he was, Theseus was certainly a very canny king and a clever politician. Between his physical power and his wits, I imagine he had no trouble getting his way in anything.