Even if the "apprentice" agrees, it doesn't make it legal. Ancient Japanese labor laws do not apply here. And even if it is legal, which it appears it may be in that state, it doesn't make it ethical.
Also, there is only a very small chance of a big pot of gold at the end of the rainbow.
This makes me wonder just how many applicants did 3 months or less of free work... only to move on or get let go... only to have the next recruit start his/her 3 months of free labor. It just screams scam to me, but hey, I'm a "western" guy with modern values, so what do I know?
Again, go for it if that's what you want, or if you're very desperate to break into the nursery business. I just don't think it's smart unless you're financially able to handle the lack of pay and absolutely know that bonsai is all you ever want to do.