Kendrick not traded to Yomiuri Giants

Yahoo featured a joke played on Phillies pitcher Kyle Kendrick about his being traded:


Philadelphia Phillies pitcher Kyle Kendrick was shocked to learn that he'd been traded to the Yomiuri Giants in Japan for a player named "Kobayashi Iwamura."
"I don't know what to think right now," he told reporters outside his locker shortly after getting the news from assistant general manager Ruben Amaro Jr.
Had he truly been thinking, he would have realized that such deals are prohibited. And with a little digging he could have found out that there is no Kobayashi Iwamura playing in Japan.
The "trade" was, in fact, a prank. The ruse was orchestrated by pitcher Brett Myers and was elaborate enough to include Amaro, manager Charlie Manuel, Kendrick's agent, the media and others.

IPBiz query: what prohibits such trades?

Merely fyi, see a discussion of the United States Japan Player Contract Agreement.