What is at question here is the Vienna Convention on Consular Relations. This multilateral 1963 treaty provides, among other things, the right of foreign nationals accused of criminal activity in a host state to have access to consular personal from his/her country of citizenship.
Article 36 (Clauses 1-3)
Commication and Contact with Nationals of the Sending State With a view to facilitating the exercise of consular functions relating to nationals of the sending State:
consular officers shall be free to communicate with nationals of the sending State and to have access to them. Nationals of the sending State shall have the same freedom with respect to communication with and access to consular officers of the sending State;
if he so requests, the competent authorities of the receiving State shall, without delay, inform the consular post of the sending State if, within its consular district, a national of that State is arrested or committed to prison or to custody pending trial or is detained in any other manner. Any communication addressed to the consular post by the person arrested, in prison, custody or detention shall also be forwarded by the said authorities without delay. The said authorities shall inform the person concerned without delay of his rights under this sub-paragraph;
The United States Constitution is very clear on the status of treaties in the
United States and its applicability to all of the States
Article VI Clause 2
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound hereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.
For all of his other problems, President Bush was right to oppose the State of Texas and called on the Justice Department to argue for upholding the treaty before the Supreme Court. However, despite the clarity of Article VI of the United States Constitution, the Supreme Court came up with the following inexplicable ruling:
Medellín v. Texas, 552 U.S. 491 (2008) is a United States Supreme Court decision which held that while an international treaty may constitute an international commitment, it is not binding domestic law unless Congress has enacted statutes implementing it or unless the treaty itself is "self-executing"; that decisions of the International Court of Justice are not binding domestic law; and that, absent an act of Congress or Constitutional authority, the President of the United States lacks the power to enforce international treaties or decisions of the International Court of Justice. (from Wikipedia)
The United States Department of State insists that all foreign governments abide by the provisions of the Vienna Convention. It did recently when a U.S. man working out of the embassy in Pakistan was held by Pakistani authorities. When China holds U.S. citizens for whatever reason, the U.S. demands to have access to them. If U.S. states begin to ignore this treaty (and this is not the first time in the case of Texas), this puts the legitimate rights of Americans overseas in grave jeopardy.