The players action had nothing to do with his scoring on the play. You don't take points off the board because the player is a no class jerk.
You penalize him so as to discourage future repeats of similar conduct.
Here's what the NCAA Football Rule Book says:
"If a player makes a taunting gesture to an opponent on the way to scoring a touchdown, the flag would nullify the score and penalize the offending team 15 yards from the spot of the foul."