Because by doing so, law enforcement can manipulate the image from the source by:

• Intercepting the payload and modifying the operative system to send data to law enforcement
• Pose as the origin of the original payload, and send the tainted operative system to other devices when they reboot

Unless, of course, the BIOS stores the checksum of the untainted image. (Which adds its own can of worms, because that would make legitimate image upgrades require writing the new proper checksum on each server)