If all people who drink a certain soda will pick the vanilla ice cream, then you can't precommit to chocolate ice cream before drinking the soda. It is a premise of the scenario that people who drink a certain soda won't pick the chocolate, and precommitting implies you will pick the chocolate--precommitting is incompatible with the premise. You'd try to precommit, drink the wrong soda, and you'd find that for some reaosn you just have to give up your precommitment.

This is just a special case of the fact that choosing an option is incompatible with your "choice" being determined by something else.