First of all, you need to understand what the "One China" policy means from the US diplomatic point of view. The USA ceased to recognize the Republic of China as the government of China when relations with the People's Republic of China were normalized in 1979.
Hence, Taiwan is viewed, albeit unofficially from the US, as a sovereign nation. The US won't intervene (according to the agreements) in the event of a Chinese attempt to seize Taiwan but there is no prevention on the sales of arms to the Taiwanese.