Why is Willis NOT in Winter Ball?

This is a bit of old news, but I've really been wondering about it.  Near the end of the season, Jim Leyland said he'd like to see Willis pitch in winter ball.   Willis' outings seemed to confirm the need.  He didn't exactly perform in a way that inspired glowing confidence.

I was impressed that Willis very willingly went down to A ball.  It made sense to get him away from the media scrutiny, but he had to agree to do it.  It was the right thing to do, sure, considering his walk-tastic, wild beyond description performances.  But doing the right thing is no gimme with athletes.  It's so much easier to just do what suits them best, or what makes sense to their sometimes over-sized egos, especially when the $ are guaranteed.  So, he gets bonus points for manning up and playing A ball.

On September 23, in a piece written by Tigers.com beat writer Jason Beck, Dontrelle said he was open to winter ball.  Then, in a Freep article published on September 29, Dontrelle was quoted as saying he had no plans to participate in any winter leagues.  It was very cryptic, with no other information given.

I've seen nothing else on this.  No commentary from Leyland or management, zip.  I'm sorry, but could someone tell me what the crust is going on here?  I would like to know why Willis is not going to winter ball, and whether there was a big dustup within the organization over it.