[cratermoon]

> Same reason why an aircraft manufacturing company would get involved in a NTSB investigation when there is an airplane crash

Key difference here is that the NTSB is third party with force of law behind it. The victims in the crash – airlines and passengers – aren't rushing to the aircraft manufacturer to come fix things. Quite the opposite: the NTSB and FAA have the authority to quarantine a crash site and ensure nobody tampers with the evidence. Possible tampering with black boxes was an issue in the investigation of Air France Flight 296Q.