Why does the shuttle throttle on ascent?

In article ,
Blueskies wrote:
: The 3 Gs is at the backs of the occupants (and along that same axis for the
: vehicle.) This is the same if they are heads up or heads down.
: If you lie with your back on the floor, you feel the same 1 G if your head
: is facing north or south.

Not talking about facing north or south, they're talking about hanging
from your feet or standing upright...

When all the forces (engine thrust and air drag) are from your back to
your chest or vice versa, being head-up or head-down is precisely the same
as being head-northward or head-southward while lying on your back on
Earth, i.e. it makes not the slightest difference in what you feel.

The shuttle in ascent is in free fall except for thrust and drag. The
ascent path, and the shuttle's orientation during ascent, are carefully
chosen to *avoid* having the wings generate lift. The wings are not
strong enough to provide any useful amount of lift during ascent, and
the dominant concern is to avoid tearing them off by overloading them.
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