Because it wasn't voluntary at all. Jim Crane bought the team from Drayton McLane in May of 2011. Of course, it had to be approved by the rest of the owners in August and it was, but it wasn't official by Selig just quite because he said he had to do some more "due delligence" which was actually Selig basically telling Crane "I'll approve you to be the new owner, only if you agree to switch to the American League. If you don't agree, then there is no sale." I think Crane fought that every way he could until he conceded because the sale wasn't final until NOVEMBER 2011. Lance Berkman had it right.

And then two years later when we're officially in the American League, Selig does this.

Not to mention there's other shit he did against the Astros and MLB as a whole - * forcing the roof to be open during the World Series despite the Astros playing the entire 2005 postseason with the roof closed for homefield advantage effect * The Cubs series in Milwaukee during the Hurricane (Drayton is also to blame for that) * Almost folding the Minnesota Twins so he could steal that market for his Brewers * The 2002 All Star Game * The Strike * The Steroid Era (if you're hardcore anti-PED. I'm not, I just think it's unfair that those players get scrutinized and Selig gets a spot in Cooperstown when he was totally cool with the money and publicity coming in thanks to those players and making baseball cool again after the Strike)

TL;DR - Fuck Bud Selig