May have something to do with Japan's non-military proliferation treaty with the US. Is it archaic? Perhaps. But, it is an existing treaty, nonetheless.
I think you're right. Interoperability with US military forces has always been a high priority for the JSDF given its small size and political limitations. From Wiki:
"The Mutual Security Assistance Pact of 1952 initially involved a military aid program that provided for Japan's acquisition of funds, matÃ©riel, and services for the nation's essential defense. Although Japan no longer received any aid from the United States by the 1960s, the agreement continued to serve as the basis for purchase and licensing agreements ensuring interoperability of the two nations' weapons and for the release of classified data to Japan, including both international intelligence reports and classified technical information."
Although there were certainly dark chapters to the story, there is also something very noble about how the US helped Japan to rebuild after WW2. I think it was also very noble of Japan to renounce war as a means of settling disputes. I know Japan is wrestling with its pacifist orientation and its sense of moral obligation to stop genocide but it seems like some people at least are trying to learn the lessons of history.