Well, given that you didn't get to choose to be a citizen... and the American government will (and does) claim jurisdiction over you, even when living abroad, I don't see anything even remotely unethical about availing yourself of the benefits of US citizenship while living abroad. If you'll have the "responsibilities" forced on you, it only seems fair that you'd be free to avail yourself of the privileges too.

As to voting... I don't see why you'd care, but vote however you want, if you want to be bothered. It's not like it actually matters, or for that matter, like everyone doesn't vote for their personal interests at the expense of the good of the country anyway. Unless you're putting anthrax in the absentee voter envelope, I find it hard to see how your voting could be considered unethical.