There are an estimated ~7 - 8million US citizens who live abroad, pay taxes abroad, and don't directly enjoy the benefits derived from paying US income taxes. All services required by citizens abroad are charged a fee, regardless of the individual's situation.
Quite simply put, it is unfair for the US government to charge US income taxes on citizens living overseas. Only the US (and Eritrea) do such to its citizens. And since our US congressmen and senators aren't in the situation to really represent American diaspora, this situation is effectively breaking the cardinal rule of the United States of America - namely, taxation without representation.