Why does the shuttle throttle on ascent?

"Dudley Henriques" wrote

I flew a D, as well as various other prop fighters as a civilian operator.
Never flew ours with external tanks. The airplane is stable on takeoff if
flown correctly and I wouldn't anticipate any specific issues with the
external tanks except the extended run. I believe the only caution on the
external tanks was for high speed buffet above 400 mph.
We had the fuselage tank removed and only flew the Mustang using the 2
mains at 92 gallons each.(90 usable)

I had always heard that the fuselage tank was the source of the instability,
with it being so far behind the CG, to give it a dangerously aft CG. Today,
in peacetime, I don't suppose they would ever dream of putting that much
weight that far back, but it was war.

Comments?
--
Jim in NC