Well, that may be so in your opinion, but that it is part of China has been US foreign policy since 1973.
The situation is significantly more ambiguous than that:
“The official position of the United States is that China is expected to “use no force or threat[en] to use force against Taiwan” and that Taiwan is to “exercise prudence in managing all aspects of Cross-Strait relations.” Both are to refrain from performing actions or espousing statements “that would unilaterally alter Taiwan’s status.”[31] The United States maintains the American Institute in Taiwan.
The United States, Canada, the United Kingdom, India, Pakistan and Japan have formally adopted the One China policy, under which the People’s Republic of China is theoretically the sole legitimate government of China. However, the United States and Japan acknowledge rather than recognize the PRC position that Taiwan is part of China. In the case of Canada[32] and the UK, bilateral written agreements state that the two respective parties take note of Beijing’s position but do not use the word support. The UK government position that “the future of Taiwan be decided peacefully by the peoples of both sides of the Strait” has been stated several times. Despite the PRC claim that the United States opposes Taiwanese independence, the United States takes advantage of the subtle difference between “oppose” and “does not support”. In fact, a substantial majority of the statements Washington has made says that it “does not support Taiwan independence” instead of saying that it “opposes” independence. Thus, the US currently does not take a position on the political outcome, except for one explicit condition that there be a peaceful resolution to the differences between the two sides of the Taiwan Strait.[31] The United States bi-partisan position is that it doesn’t recognize the PRC’s claim over Taiwan, and considers Taiwan’s status as unsettled.[33] All of this ambiguity has resulted in the United States constantly walking on a diplomatic tightrope with regard to cross strait relations.”