Yeah, I know. It seems crazy -- and while it's a backstabbing move, they do have a technical point, and have been backed into a corner of sorts. The former CEO of AIG filed against the government on the basis of the 5th Amendment last year, and has recently made inroads in convincing AIG to join the suit. It's financially intelligent for them to do so, but the PR would be an unmitigated disaster.
(Related article in which lawmakers weigh in on the anti-join side, posted by Captain_Ozone. I would read mine first and then his for a full picture.)