The problem with tying encryption to the serial number is that it becomes harder to distribute the material in the first place - either you need to know the ser

The problem with tying encryption to the serial number is that it becomes harder to distribute the material in the first place - either you need to know the serial number of the device you're going to put a drive into so you can pre-encrypt the data, or you need to have extra code to encrypt the data on first boot (and then make very sure that first boot happens in the factory). Moving to a static key makes it much easier to handle all of this. Anyone who's able to reverse engineer the system well enough to obtain a static key would probably be able to figure out the dynamic algorithm anyway, so the practical outcome isn't terribly different.