What gives China the right to prevent the Taiwanese from self-determination? What makes you think Taiwan should be part of China if most of Taiwan doesn't want to be? Taiwan has been a struggling democracy for 40 years and developed its industry separately from the communist government.
This is a legal issue based on international law. Taiwan up until about 10 years ago claimed to be part of China and the political leadership claimed to represent all Chinese in the mainland. The Kuomintang still claims to represent all of China.
So, in the eyes of the world, how can the Taiwanese renounce something they claimed all along? If Taiwan wanted to break away from China, it should've done so a long time ago.
BTW, the USA (and most of the world) says that Taiwan is part of China.