Most of the world doesn't accept the SCS as China's "territorial waters".
China often invokes the Law of the Sea Treaty to justify exploiting the area, but what China did when it signed was
attach a letter to the treaty saying that China's boundaries are whatever the government says they are - in direct contravention to much of the content of the Treaty. Then, when many countries complained about this grab for territory, China doubled down and declared it would not abide by the conflict resolution procedures of the Treaty, either.
So China wants the fruits of sea diplomacy but few - if any - of the duties. I imagine the purpose of this thuggish behavior is to provide the Party with a mechanism to begin a war at its convenience, providing an excuse for wartime unity when its own domestic situation becomes shaky.