Sixth Amendment Preservation Act This bill amends the federal criminal code to limit the detention authority of the United States. Current law prohibits the indefinite detention of a U.S. citizen by the United States unless it is authorized by an act of Congress. This bill prohibits the indefinite detention of any person by the United States unless it is consistent with the Constitution. Additionally, it specifies that a general authorization to use military force or similar measure, on its own, does not authorize the indefinite detention of a person who is arrested in the United States. Finally, the bill amends the National Defense Authorization Act for Fiscal Year 2012 to repeal a provision that affirms the authority of the Armed Forces to detain individuals captured in connection with hostilities pursuant to the Authorization for Use of Military Force pending the end of hostilities.