The Japanese ultimately adopted it. The way America dealt with the vanquished was almost unheard of in the annals of history (and is probably why Japan and Germany have become such successes). It is as voluntary as most constitutions are.
Viz. treaties, if it's really imposed by the victor then it's not a treaty, so the element of imposition does not differentiate the Japanese constitution from a treaty.
Yes, we treated them well. And it worked out well for history.
But if you think treaties are not often/most imposed by force..including victories. Well ok, you are welcome to that belief. Sometimes treaties are negotiated for common good. I'd wager that almost all, however, are dictated from a position of strength thru out history.
I'll maintain that a Constitution agreed to by someone you just nuked(and it was justified) and parked your battleships in their harbors. Really is not as valid as one freely entered into as was ours. Just my thoughts on the matter.
And you are welcome to the last word as well on this.