The 14th Amendment's first sentence reads:
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.
It is accepted that citizens of the United States (including native-born citizens, and persons that hold only U.S. citizenship) are allowed to renounce their citizenship. For example, Facebook executive Eduardo Saverin recently renounced his U.S. citizenship (leaving him with only Brazilian citizenship).
Be it enacted[...], That any declaration, instruction, opinion, order, or decision of any officers of this government which restricts, impairs, or questions the right of expatriation, is hereby declared inconsistent with the fundamental principles of this government.
But this seems to contradict the plain reading of the Citizenship Clause, which appears to state that citizens are citizens regardless of whether they purport to have renounced their citizenship. So what is the Constitutional basis for allowing natural-born U.S. citizens to renounce their citizenship?