The aspect of Japanese social games that the agency is rumored to disapprove of is the "complete gacha" style of microtransaction, in which players pay for randomized items, earning additional in-game rewards for completing a set. This could be construed as a form of lottery, which is illegal as a sales incentive in Japan.
GREE officially declined to comment to the Wall Street Journal about the potential impact of a ban on these microtransactions, noting that there were other ways to build revenue. However, we suspect Tanaka himself has a different view on the potential financial impact, having already seen it happen. GREE and other social gaming companies have announced plans to self-regulate their games, creating industry guidelines and imposing spending caps.
This (what's the opposite of a windfall?) happened just a week after GREE announced its purchase of Funzio, developer of social games including Crime City.