Or rather what the US want, considering the same push was supported by Obama, GWBush and Bill Clinton, and would have likely been supported by Hillary Clinton as well. The reason is simple: money. The realities of US debt are such that defending multiple countries “for free” looks more and more economically difficult with each year. US diplomacy has focused for almost 20 years on trying to get allies to share more of the burden; but to increase military spending, allied governments need political cover, and the only one currently available is peacekeeping on foreign soil. You cannot do that effectively if your Constitution explicitly forbids part of it, so here we are.
The same happened in Germany during the Balkan intervention, btw - constitutional blocks had to be removed so that panzers could again roam over Europe, saving the US a very expensive deployment. It’s a constant refrain at any NATO meeting: “when will you spend what you promised to spend? When will you help on this and that? Because we’re a bit squeezed here.” (for the record, the argument would look better if only the US had not wantonly burnt bazillions of dollars toppling Saddam and occupying Iraq - but hey, that goes for pretty much any argument in US foreign policy since 2003).
I would criticize Abē for being as hopeless about the economy as his predecessors, for all his shenanigans, but I honestly don’t see any real “Trump connection” here. Bar adopting a policy of universal and unilateral disarmament, US and Japanese interests simply align very closely at this point in time (and over China, they also very much align with Russian interests).
