Charges can only exist on the basis of legal technicality. That’s how law works.
What you are describing is guilt by association with China. It is not illegal to work to promote scientific collaboration with China. It is not illegal to participate in advancing Chinese scientific interests. Some people might consider these things immoral. Some people might not.
Clearly the prosecutor felt as you do, that because Dr Chen’s work involved advancing Chinese scientific interests, he must be some kind of spy or traitor. If you remove the preconception of guilt by association with China, you might realize it’s possible to want to do science, even science funded by a foreign country and advancing that countries interests, and not automatically be some kind of traitor or spy.