I translated a few pages from Japanese, I saw mention of a 10mw power limit on 2.4 GHZ in Japan.
Pitstop000, I advise you to contact Horizon/Spektrum yourself for the information you seek. They were the ones who supposedly told you it was legal in the US, I am unable to confirm this and I'm not looking anymore.
About the only thing I am confident of is that DSMJ is NOT compatible with DSM or DSM2, I will also note that each country has their own laws regarding the 2.4GHZ band, If you operate something in the USA that is designed for a different set of laws than ours, you would be assuming a lot of liability.