Why does the shuttle throttle on ascent?

Henry Spencer wrote:
If memory serves, Boeing eventually concluded that this wasn't a good way
to do it -- too much structural strengthening needed? They looked instead
at injecting fuel into the bypass ducts of the turbofans (!), and
concluded that they could get a very large thrust increase that way, and
that it wouldn't hurt the engines if you kept the duration short.

Somewhat similar to the way NASA looked into upping their SR-71's
performance via H2O2 injection into the afterburners.
But in that case the improvement in performance was pretty marginal.
Borrowing a page from the D-21/M-12 program, I wonder if you could use
the mini-shuttle's engines for the final climb, while constantly topping
the ET up from the internal tankage till release.

Pat