My guess it's for the follow on lawsuits.The guy was hit by a plane. Why waste taxpayer money when you already know the cause? Do they think someone was expecting the plane to hit him so that they could murder him the instant before?
For example, if Mr. Jogger died of a heart attack at the same time or shortly before, then his death was not due to the airplane and thus there is no lawsuit against the pilot. The autopsy will determine the cause of death.
Again, just a guess.