You are ignorant of Chinese Imperial history. Tibet paid some sort of tribute to many Chinese emperors. Chaing Kai-Shek tried to exact a tribute but he did not have the muscle.The Great Wall was to protect Northern China from Mongolians, not Tibetans. China has four times the population of the US, so it has a better justification to expand.
Apart from the monks, most Tibetans are at least as well off under China as under all the previous Dali Lamas. China has brought roads and modernity to Tibet. China could easily have conquered Mongolia and Siberia, but did not attempt to do this. The Russians moved in rather easily.
The US has dominated the Caribbean forever, except for Cuba from 1959. Navassa, Mona, and several other islands were grabbed because the US had the better navy. The sane is true of many Pacific atolls.
It seems to me that the Chinese turning barren reefs into actual islands entitles them to put a base there. Certainly if the US can hold Guantanamo against the will of the Cubans and shell Vieques for sixty years against the wishes of the people of Vieques. Vieques is essentially a part of the Virgin Island group, but it was seized from Spain along with Puerto Rico.