This is kind of odd, in that Carstens isn't Napolitano's counterpart, his job is only secondarily security-related, and he has other rather important things to be worried about. Who knows exactly what the circumstance were leading to the agreement, but sticking the (round peg) finance secretary in the (square hole) security ambit certainly feeds the perception that the US is overly concerned about security.
Second, I wonder a) How many binational agreements like the above I've read about in the past five years, and b) how many of them have been utterly worthless beyond the photo op they generated. The second question isn't entirely cynical; some of these agreements are of course significant, but I can't imagine they all are. So what is the actual ratio?