De, I'm aware of that, but Japan changed its laws a couple of years ago, to reflect new realities in the region...
Japan to allow military role overseas in historic move, 18 September 2015
http://www.bbc.com/news/world-asia-34287362
Japan sends biggest warship to protect US supply vessel, 1 May 2017
http://www.bbc.com/news/world-asia-39768110
Update - Another, a bit more informative:
2-May-17 World View -- Japan's largest warship, the JS Izumo, will escort and defend a US supply ship
http://www.generationaldynamics.com/pg/ww2010.weblog.htm