China doesn't recognize the tribunal's jurisdiction in this case, since China previously opted out of binding arbitration on territorial issues, as allowed by UNCLOS.

> Only one of those countries has been building / extending artificially reefs / islands in this area.

At the same time, the US has been conducting provocative military maneuvers in the South China Sea. These days, China and the Philippines are cooperating to some extent in the South China Sea. The US has been prodding the Philippines to take a more confrontational approach.

> China has deemed it very, very important to seize this area.

I think there are two issues. They view the sea as strategically important, and don't want to be at the mercy of the US Navy, which could shut down a lot of Chinese trade in the event of a conflict. They also don't want to lose face, and giving in to US demands that they drop China's traditional territorial claims would not go over well among the Chinese population.

> You think it's reasonable to respond to freedom of navigation operations (also conducted by other countries than the USA) by militarising islands that aren't even theirs?

"Freedom of navigation operations" is a propagandistic name given by the US Navy to very provocative military maneuvers in what China views as its territorial waters. I think it's entirely understandable that China puts defensive weaponry on islands it considers its own, in response to perceived violations of its sovereignty by a hostile military.