It is not illegal to decrypt Apple's private signing key and redistribute it. Look at the private key Apple uses in the Airport Express to encrypt AirTunes. It was extracted from the firmware about a year ago and reditributed on the internet. Apple has yet to sue the person who extracted it, update iTunes to blacklist that key, update the Airport Express to use a different, or anything else that even suggests that they care about that key being public. Bottom line, it is not illegal to decrypt Apple's SHSH blobs to extract the key(s) used for signing them.
Just because they didn't care to pursue legal ramifications in your example, it does not make it legal to decrypt and redistribute private keys, if not then they would not be called private