If they are truly blade servers, then they were possibly sharing the same chassis, power supply and backplane. Could the FBI have pulled just the blades in question? Possibly. But I can very easily imagine the entire blade chassis being viewed as a monolithic component that they would want to be able to perform whatever forensic analysis they are planning. They could also have pulled whatever blades they were not after, and left them, but until you replace the chassis, you are dead in the water.