Same reason why an aircraft manufacturing company would get involved in a NTSB investigation when there is an airplane crash. Just because they messed up one or more things (i.e. MCAS on MAX) doesn't mean they can't provide expertise or additional resources to at least help with the problem.
Your take also casually disregards the fact that Delta took an extraordinary time to recover from the problem when the other companies recovered (albeit slowly). This is the point that I'm getting at. It isn't that CS and MS aren't culpable for the outage; it's that DAL also contributed to the problem by not adequately investing in its infra.
> 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.
Your take also casually disregards the fact that Delta took an extraordinary time to recover from the problem when the other companies recovered (albeit slowly). This is the point that I'm getting at. It isn't that CS and MS aren't culpable for the outage; it's that DAL also contributed to the problem by not adequately investing in its infra.