my impression is that the japanese seem to particularly loathe the notion of chinese and russian retribution. to submit to either equates to the undoing of whatever victories they had against them (e.g. the victory at tsushima, so central to japan's notion of peerage among the powers), and in the process, a lowering of their status amongst the 'hierarchy of world.' it is notable to me for example, that in the last days of ww2, despite americans killing far more japanese than russians ever did, japan favored surrendering to america rather than the russians who had already pushed into manchuria.
so with that being my baseline assumption, how would the japanese see themselves best avoiding retribution? if they believe that, without american protection, that russian and chinese retribution are merely a matter of time, then wouldn't they still feel more incentivized to aid the us, because that is the only chance they would ever have at forestalling retribution?