A pro-China Taiwanese president would greatly increase the chances of peaceful unification between Taiwan and China, which would basically let China get a hold of F-16s (albeit earlier Block), Various advanced cruisers, Patriot missiles systems, and what not. Even if it is downgraded compared to current US inventory frontline fighters, it would be a major "inconvenience" for the US if China was able to closely examine them.
In fact, something worth mentioning about this exercise is that Taiwan agreed not to fire the AMRAAM because of US pressure... it's a little paranoid, but understandable, I guess. (Even if the chances of China learning anything from an AIM-120 launch are minimal)
Edit : Added picture
Interesting picture from the psychological warfare exercise.
Basically it says (in simplified Chinese, so I assume it would be directed at PLA troopers) Put down your weapons > Take off your helmets > Raise your hands.
Surrender to the Republic of China
We will ensure your safety