At the decision point in a counterfactual mugging, you already know the coin has landed heads. The only consequence of your decision to pay $100 that TDT cares about is that you pay $100. That your choice to pay the $100 counterfactually would have resulted in Omega paying you $10000 if the coin had landed tails doesn't move TDT, because that consequence is counterfactual, that is, it doesn't really happen.