if a child's drowning in a pond, and I tell you I'll only fish him out if you give me $1,000, it seems like I'm blackmailing you.

Perhaps a borderline case like this is most helpful. Is this extortion? Even though the default case in this case isn't 'doing nothing'. The default is saving the child. Because that is what someone should do.

So maybe the word is difficult to unpack because it has morality behind it. A person shouldn't bomb your car, and shouldn't expose your private secrets. On the other hand, they needn't give you food, so it's OK to ask for money for that.

If I demand money for being faithful to my husband, than that is extortion because I'm supposed to be faithful. If, however, I want a divorce and would divorce him, I'm allowed to let him pay me for faithfulness. Such gray areas indicate to me that it is indeed about some notion of expected/moral behavior.

Selling food to starving families -- when they become so poor that you ought to give them food for free -- then that is extortion.

So: demanding more compensation when you should do it for less (or demanding any when you should do it for free).