Maybe BMW-Sauber didn't want to have to deal with Ecclestone and the FIA anymore and, since FOTA wasn't starting its own series and a new Concorde Agreement was to be inked, which would have further obligated them to Ecclestone/FIA for some time, they just opted-out of all of it. :shrug:
I kind of also wonder whether they maybe took exception to the fact that there are no longer any North American races, since that is, if not the largest, one of the largest markets BMW serves. At that rate, what's the point? I'd probably just concentrate on my ALMS (DTM, BTCC, etc.) efforts and save my money in the process, too.