Turn it inside out: Let's say an American citizen is accused of a crime in a foreign country. Would we prefer that the American have access to the US Consulate before trial? I would.

In order for international law to work there must be willing participation of signatory countries or a real and credible enforcement mechanism. Since the latter requires the willingness to employ an act of war, usually the former is a less dreadful path.

Twitchkitty makes a fair point, too. But carried to all international treaties, it becomes a disorganized mess in which each state of the United States decides whether or not to follow the national will through treaties signed by the president and passed by the senate.