This would not last very long of it was made into a law since it's considered to be unconstitutional. Even those that commit treason do not get their citizenship revoked. If that citizen verbally revoked his or her citizenship while not under pressure to do so then they are revoking their rights a as U.S. Citizen and therefore are not bound by the laws of that country anymore and that country is no longer bound by the constitution to not treat them as an enemy combatant. Mind you that person would also have to sign documents and swear that they are not revoking their citizenship because of tax purposes. This applies for naturalized citizens and those that were born on American soil or to at least one parent that has U.S. Citizenship. It would be immoral and unethical to pass this law since it set a precedent for future use of natural born citizens.