Maybe so, but if you knew before hand that the BoJ was going to be buying large quantities of Yen at a certain time, there would be an obvious trade there. It might not be 100% certain to make the yen rise, but you definitely have an edge knowing that ahead of time and going long Yen.
This market is so huge and people have so many reasons for buying and selling, that it would be almost impossible to prove even if it was illegal.
There are not any insider trading laws regarding forex or commodities. Its perfectly legal and part of trading these instruments.
The below forum is a good source of information like this: (for future reference) http://www.seclaw.com/ubbcgi/Ultimate.cgi
Yes he was investigated, but if I recall correctly it had something to do with stock in France. It definitely was not about currency trading.
Which I suppose makes TA even more important for trading them because you can be assured some big player always knows more than you do.