Booting from a CD would [let you replace the infected BIOS](http://www.flashrom.org/Live_CD). Restricting the boot devices presumably helps preserve the BIOS infection.
Fair enough, presumably it's easier to simply prevent booting from CD than subvert every tool for flashing BIOSes.
In the end, it seems to me that blocking boot from CD is a net loss, since it alerts the user (who can undertake drastic measures including hooking up a different hard drive), whereas allowing boot from CD but reinstalling the BIOS malware from the (presumably thoroughly infected) hard disk would not.
