Helping a strategic trade ally and making it clear that they have the backing of the US in more than just words seems to me like something that would make invading Taiwan even more risky than an amphibious invasion would already be. It's not like Taiwan (or the US) is going to invade the mainland, so I can see why this is, and has been, the foreign policy of the US. The US aircraft carrier group that's patroling the area and the commitment to defend Taiwan in the TRA are already a thing. This is just following through on commitments already announced. I don't see a way that this transfer of weapons could be used as a pretense for an attack where the international response wouldn't be extremely negative towards mainline China. I don't agree with a lot of the foreign policy of the US, but I can see how they justify it with their own interests.