I mean it's not meant to be a surprise. It's been the official policy of the United States since 1972. Specifically "the United States acknowledges that Chinese on either side of the Taiwan Strait maintain there is but one China and that Taiwan is a part of China. The United States does not challenge that position." https://en.wikipedia.org/wiki/Shanghai_Communiqué.
It's only a surprise because the US treats treaties like it says "as long as the grass grows or the water runs" https://en.wikipedia.org/wiki/Indian_removal