They aren't going to announce a fenway show, at least not this year. They are playing this show because Wrigley is making a routine out of summer concerts and eddie loves the cubs.
That's what I originally thought, but after I heard the rumor, it kind of made sense. The Red Sox old general manager, Theo Epstein, is now the Cubs general manager and he has been spotted numerous times at Pearl Jam concerts, so he is obviously a big fan. That just seemed like too big of a coincidence for me.