The terrorist iphone thing seems to be asking for something that Apple could actually do, meaning knowing a specific encryption key is not necessary... not if Apple could comply.
So what the FBI should be asking for is for Apple themselves to take the phone, and in private, do whatever they need to do to access the email on that one phone, and to provide that one specific result to the authorities. More like a specific search warrant then, which if the comparison holds, our laws do allow courts to order for specific named searches.
But they should not be asking for Apple to provide the means to get into any future phone for any reason, by anyone holding the method. That definitely would Not be a specific court ordered named search with probable cause.
However, I suspect it would mean that Apple could then set up a new department, just for such searches.