I think this is a terrible idea.
1. They aren't operationally required in Japan in any way, shape, or form. US air assets in Japan are already formidable, not to mention the Japanese Self Defense Force is a pretty potent military organization as it is.
2. The F-35B still has quite a few significant software issues before coming 100% online. Declaring it operational was more for PR purposes than anything substantial.
3. And this is the big one....WHY do we continue to give our potential future enemies as much intelligence as we do, about some of our most significant capabilities!?
China will be probing Japanese airspace every chance they get. Logging, recording, studying every single radar contact they have with any F-35B they encounter. Learning from it, developing doctrine, modifying weapons, modifying radars, etc etc.
Would it not be in the West's best interest to keep our cards close to our chest, until the day comes when we need to say "Oh hey, look at all the things this plane can really do!"
Seems arrogantly counterproductive to give the Chinese a chance to gather as much intel as they can, during a period of uneasy tension in that region of the world.