It's a complex tale of history and geography. 300 miles from land is designated a countries waters. But with so many countries so close, and disputed islands (notice how china and Vietnam both claim their sea wraps around the same islands?) there nonsollution that doesn't result in overlap. Considering that the US isn't likely to restore sovereignty to Hawaii, give back native lands, or return the southwest to mexico, we start at a disadvantage to point fingers. Complicating it further, 50~ years ago the US suffered a great defeat while trying to keep the communist and current government from taking over Vietnam, and the current government fought to keep the US out of their affairs. It's not right but if you're "strong" you can do whatever you want until someone stronger stops you. The poll becomes: are you ready to go to war with China to try and stop them for those countries benefit? That's the simplest way to state the situation.
Im not saying who's islands they are. There are quite a few people who believe those islands should be theirs and I'm not here to say anyone is wrong. I'm just saying that the Philippines already took this to the UN. The UN already told China that. China told the UN they didn't like their decision and weren't going to abide it. In other words they said "make me." The UN Isnt going to make them, Taiwan can't make them, the Philippines can't make them, Vietnam has already lost 2 fights over the islands to them. So whoever "owns them by right" China owns them by force, and is willing to use force to keep them until someone compelled them to play nice. There are only a few individual countries that can do that- US carriers have been used for instance. The common narrative being that the US are "world police" getting involved in other countries affairs- but when a huge country like China decides to pick on a small nation that can't fight back- is it better to just let them? It's complex.
The UN has no teeth. It's marginally better than the League of Nations, and some good comes from it- but when things get serious the UN is about as effective as a cocktail umbrella in a shit storm. To quote aliens: "what are we supposed to use? Harsh language?" It's basically just a "mean girls" clique to go gossip and try and sit at the popular table while keeping the "weird" and "poor" kids excluded but making people think you're being nice to them.
It's ours and many other countries'