Senators, Russians and allied leaders who insist that the U.S. must not depart from the ABM Treaty understand full well the practical effect of this arrangement.

CENTERFORSECURITYPOLICY: Which Bush on Missile Defense?

One reason perhaps is that although the Russians seem to be keen on such a regime, the US has not elicited enough concessions in principle on law enforcement to make this worthwhile from their perspective (notwithstanding the probable lack of practical effect of such an agreement also).

FORBES: A Network Ambassador for a Networked Superpower