[Koda94]

so if you had to engage or chose to engage the mall shooter, would it not be ideal to reholster your weapon immediately after the threat is stopped?

to break this down; realistically your either going to be immediately involved or your going to have to make a choice to get involved and engage or retreat. The latter affords your best bet to retreat to safety there is no need to draw any weapon putting you in jeopardy of being mistaken by LE or other CCW civilians.