He's not fully joining the fight against Obamacare - Koster has issued an amicus curiae brief in modified support of the Florida Obamacare lawsuit, instead of joining it - but he's easily the most prominent Democrat on the state level to break with his party on Obamacare. This is partially probably due to Missouri's emphatic rejection of Obamacare last year (via Proposition C), and partially probably due to Koster's own desire to survive politically; Koster switched parties in 2007, when it looked like the promised forty-year dominance of the Democratic party in America might actually last, well, forty years. As it stands, Koster is up for re-election next year, and as it's promising to be a bad year for Missouri Democrats who like Obamacare... well.
Do read the brief, as it represents the Democratic party's somewhat frantic desire to resolve the problem that they're having with Obamacare right now. To wit: the individual mandate is clearly unconstitutional, given that it requires people to engage in commerce, whether they want to or not. Unfortunately, the same Geniuses From Beyond Space And Time that put that provision into Obamacare also neglected - willfully - a provision that explicitly stated that the various parts of Obamacare are severable from each other; so if the individual mandate goes it's well within the court's purview to declare the whole thing unconstitutional as well. That would be... problematical for Democrats, given that they wasted a year on Obamacare in the first place.
Which is why Koster spent so much time in the brief arguing against the mandate, and only briefly argued for severability: he's got a strong case for the former, but not so much of one for the latter. Indeed, the Justice Department has argued in other Obamacare cases that the lack of severability was deliberate: for that matter, the Vinson decision that Koster is now offering advice for explicitly ruled that the individual mandate is not severable, which practically guaranteed that the case will arrive before the Supreme Court eventually. Short version: a lot of people expect that the individual mandate will be excised*; the big fight is over trying to either expand or contract the implications of that, and AG Koster really isn't on the right side of that one.
He's just trying to carve out his own little exemption from the consequences of passing Obamacare.
Moe Lane (crosspost)
*Which does not mean that it will happen. A lot of people expected that McCain-Feingold would be thrown out as being blatantly unconstitutional, too.