Here, the US plane appeared to be acting legally by flying over and collecting information. Clearly if it caused the collision, that is illegal. If the PRC caused the collision, that is likewise illegal. It issued a mayday call and then proceeded to land in Hainan. As I noted in the last message, it seems that even military planes have the right to make emergency landing without prior permission.
Criticisms are welcome.
However, here the reason would be the breach of China's airspace. Again, I am not sure.