Followup question: Why was Mitsbubishi allowed to continue operations? Did other Japanese manufacturers have the same fate as IG Farben? My instincts tell me that we didn't want to cripple their industry but I don't see why there wasnt at least a little "rebranding".

Did it have to do with whether or not they were active pre-expansionism?

While it is true that many companies did work with the Nazis and use slave labour, IG Farben was singled out more for its role in the Holocaust and the production of Zyklon B.

Note however that after the war the original idea was to simple run down the entirety of German industry, went this proved impractical and with the upcoming Cold War it was decided to let West Germany rebuild. It was easier to restart pre-war companies and factories rather than build new ones.

Some were put back in business even to wake weapons, like Krupp, which even had its owner released from prison to make weapons for the US for the Korean War: http://spartacus-educational.com/FWWkruppA.htm

Otherwise Germany was just too important economically to Europe and the US to allow its industry dissolve; the disruptions were already too much to bear post-war, so they had to accept that the organizations were going to continue and some of the C-level executives be prosecuted. Even that was often too much and they had to just make peace with rehabilitating Germany and letting a lot go unpunished other than a couple/few year prison stint.

IG Farben specifically was made an example of because it was just too big, unwieldy (part of the reason it was broken up was that the US was getting rid of the German cartel system), and influential. The other companies were too small to worry about breaking up, rather than IG Farben, the largest cartel in the world, or at least Europe.