because investigations performed by the companies or governments themselves tend to portray themselves as not culpable for any actions (often omitting pertinent details). Independent review is the only way to be honest.
China and Apple have their reputation to protect, so they have a very high stake in the game.
The Tech Transparency Project's goal is to uncover misconduct and malfeasance in public life. They don't have to 'prove' any wrongdoing with regards to Apple or China. If their research with regards to Apple or Chine doesn't pan out, they have many, many more potential targets without danger of losing any funding at all.
Do you have any serious evidence? Otherwise we are raising random possible problems. Maybe it's a false flag! Maybe it's the CIA or FSB or Martians! Maybe! Not necessarily!