Half of the thread's discussion is about if keeping it for yourself or spending it for something else is morally fine, when you don't know if the owner still have access to that money. Not spending the bitcoin might be morally incorrect as well, because it could be used to something that's morally correct.

As said by Welsh, it's some sort of moral dilemma similar to the train tracks. Spending money that isn't yours, ethics asides, is theft.