I want to know if in Japan I need an adapter for my USB Wall Bricks.
The answer is
(One interesting point - no such 'adaptor' exists! Because there is nothing to adapt.)
The whole issue can be instantly resolved by, obviously, glancing at the specs printed on power plugs, where one will instantly see they (of course) operate perfectly in all of Japan.
Just as the OP mentions, it's one of those curiosities where the interweb is filled with misinfo.
I'm adding here the two most important comments on this page, which say it all:
"That answer is very misleading. Can you point to ANY device that works in the US but not in Japan ? I've worked in electronic product development for 30+ years and I have never encountered something that wasn't compatible"
Key point highlighted.
"You absolutely do not need any adapters. I go back and fore between the two countries (with literally cases of equipment) all the time. Any other information you hear, is wrong."
OBSCURE CAVEAT !
It occurred to me that. Let's say you are, perhaps, an audio or video engineer and or indeed a person who manufactures electrical equipment or such. It's possible that part of your life is using things like this ...
of course, obviously, such things that literally manipulate the electrical supply and are used by electrical supply engineers are irrelevant to this discussion, and some models only work with one very specific power source, or whatever.
Of course obviously you can't casually use such stuff unless you know what you're doing, and it's inconceivable any power engineer, etc, would be asking the question on this page.
(Note that, as a curiosity, the thing in the photo if you ! merely plug two of them in series ! you've just done an incredibly dangerous thing; the issue is completely outside the question "Are Japan/US power blocks interchangeable, answer Yes".)
Excuse my incredibly ugly carpet BTW.