This underlies one of the core questions of philosophical ethics: "what should you do?" There are many moral frameworks* but the way I've always understood it, underlying each of them is that morals are defined by the obligatory feeling they give rise to. (Those feelings may arise regardless of who or what created a particular circumstance.) So for someone to find a situation immoral but not to act is to be immoral himself (or to lead him to rationalize the moral dissonance by revising his moral framework so he perceives that situation as moral).

* E.g. dominant frameworks are: morals are that which is necessary as a matter of integrity and self-respect, morals are that which you would prefer all people strictly adhere to, morals are that which provide the greatest good to the greatest number, morals are that which promote the state (or increase compassion, or knowledge).