You're assuming that their performance is directly proportional to the performance of the market as a whole. My understanding is it's tied to how they perform a

You're assuming that their performance is directly proportional to the performance of the market as a whole. My understanding is it's tied to how they perform against an index like the S&P 500. The S&P 500 lost something like 50% over 2008, so even if a trader lost 25%, they outperformed the market by a lot and deserve (according to their contract) the bonus.