Major League baseball started getting noticed in Japan when Nomo first went over, but it became huge when Ichiro and Matsui (the two undisputed superstars of the Japan League) joined. NHK now broadcasts at least a few games every day, and game results, at least for teams with Japanese players, are regular parts of the nightly sports news. As a result, lots of people in Japan are watching the games on TV, and a lot of companies are buying ad space just to target those viewers.
I may have just been seeing blue-screen ads than only show up on TV (and so could be arranged by market), but I remember seeing ads in Japanese all last year in the Kingdome and Yankee Stadium, for Ajinomoto Foods, Nintendo, JAL and a few other companies. On the Yomiuri billboard, the number for US subscriptions was probably added as an afterthought, with Japanese viewers being the primary target.
And in case anyone was wondering, the Yomirui Shimbun is the owner of the Yomiuri Giants, Matsui’s former team.