Rumsfeld had ordered the plane to be shot down,
and they were literally seconds away from doing
so..

I suspect they felt that they had to say this.

I also would guess that they had no real intention of shooting it down,
especially after almost screwing up with the Kentucky governor's plane,
and because it was just a Cessna 150.

Imagine the political fallout if they shot down any innocent plane
whatsoever, especially if that caused more injuries on the ground.
After that, it would be even more difficult to do anything against a
non-obvious threat.