Huh? I imagine is the general reaction to that entry, but it's true anyway. This is an accidental flash point in a serious argument over the extent of sovereignty and the nature of naval control.
It's a generally accepted principle in the western world that bodies of water which are not completely enclosed by a country are not controlled by it (except for a buffer zone along the coast of the country, whose width varies according to the intent for which the control is claimed and the treaty under which it is claimed); instead, it is an international free zone, open to vessels from all countries, and under the explicit control of none. The US could not claim soveriegnty over the Gulf of Mexico, nor could any of the countries bordering on the Mediterranean claim it as their own; even in the Caspian Sea, which is for all practical purposes completely under the control of Russia; the Russian Federation does not dispute the principle that it has no exclusive rights over it.
China's view, at least as applied to the South China Sea, is different. Since ancient times, the sea has been considered by the Chinese to be part of its territory. This has been effectively true at some times, and effectively not true at other times -- but, like the Chinese claim to sovereignty over Tibet, the claim has never been relinquished, and is still psychologically important in terms of the way people conceptualize *China*. The fact that other countries treat it as though it were international waters is, in Chinese eyes, a little bit like the German conception of the interwar status of Danzig: a territorial acquisition imposed by foreigners on the country without its consent (even more so in this case; Germany agreed to the interwar status of Danzig, but China has never signed a treaty which abrogated its claims to the South China Sea) and enforced on an unwilling population by force of arms.
So a US plane goes down over the South China Sea and lands at an airbase in China. (I'm leaving aside the details of the collision, as only the two militaries really know what happened, and anything anyone else says is speculation). China immediately demands an apology -- an apology for infringing Chinese territorial rights by flying a spy plane over Chinese waters. The US refuses -- as far as the US is concerned, its plane was comfortably in international waters.
The problem here is that, while it appears that all the two countries are arguing over is an apology, it goes deeper. The US can't apologize without in effect conceding the Chinese position that the South China Sea is part of Chinese territory -- which it, for economic, political, and military reasons, is not prepared to do. (It would irritate the other countries that border the South China Sea and claim islands within it; it would impose a theoretical risk to international shipping in the area by opening up the possibility that China could control which ships go through the sea; it would create an exception to a general international rule that oceans are international territory, providing a precedent for the breakdown of the rule elsewhere). Nor can China easily withdraw its demand for an apology; its territory was violated, in its view, by an arrogant bully -- and withdrawing the demand for an apology concedes the point that the South China Sea isn't its territory, which would be a massive concession almost unthinkable by a resurgent nationalist power.
So the diplomats must search for a way out -- while opinions are hardening on both sides. It looks to be a long standoff.