A quote from a different forum will help you all understand part of the reason why this choice was made:
Originally Posted by rage20980
Ferrari spends around $450m-$500m on their F1 team. They also pull in the same amount in sponsorship, prize money, TV money, etc. Ferrari AFAIK operates their F1 team at a small profit.
BMW's target was to win the F1 championship this year. They're not even close to doing it, so they called it quits. BMW won't be pulling in the dollars like Ferrari, so they're probably operating at a loss of under $50m. Their board never mentioned the economy in the press conference, just poor performance, and realigning BMW's image towards "sustainability and environmental compatibility", whatever that means.
Honda's pullout last year was a mix of poor performance and $. Honda had no sponsors, so they were probably losing $250m-$300m a year!
And don't think that just because BMW is dropping F1 Sauber that they are leaving their racing heritage in the past. They still have the Gt class races and with this much more money to spend, this will allow them to focus that budget to various other places