"Has a law been passed or judicial decisions been made which actually state that is so?"
Yes it Is an established principle see here what an EFF lawyer says on it, (extract below):
"Reengineering iOS and breaking any number of Apple’s promises to its customers is the definition of an unreasonable burden. As the Ninth Circuit put it in a case interpreting technical assistance in a different context, private companies' obligations to assist the government have “not extended to circumstances in which there is a complete disruption of a service they offer to a customer as part of their business.” What’s more, such an order would be unconstitutional. Code is speech, and forcing Apple to push backdoored updates would constitute “compelled speech” in violation of the First Amendment. It would raise Fourth and Fifth Amendment issues as well. Most important, Apple’s choice to offer device encryption controlled entirely by the user is both entirely legal and in line with the expert consensus on security best practices. It would be extremely wrong-headed for Congress to require third-party access to encrypted devices, but unless it does, Apple can’t be forced to do so under the All Writs Act."