Typically because the cost of not killing the hostage takers is basically the risk of dead hostages. There's nothing else at stake. You can safely assume that the next plane full of hostages that has terrorists at the controls will be shot down.

I'm not really advocating yes or no to bombing hospitals or schools to kill terrorist leaders hiding within - but your assertion is false. We will kill the hostages. All actual breaches involve a risk of % losses and that's baked into the decision to go in. Just a person somewhere trying to make a decision about the best outcome, for the "greater good".