Originally, I thought that the FBI had every right under the All Writs Act to compell Apple to provide the necessary (what is the difference between a front door and a back door in telecoms parlance please) because there was no existing law which covered the problem. However, Apple lawyers have drawn attention to Calea provisions which says that telecommunications providers are covered by Calea. Apple not only provides mobile phones, but they also provide instant messaging, so they are telecommunications providers, and therefore covered by Calea. Therefore, ipso facto, because there are provisions within CALEA to cover this very problem, (or not), then the All Writs Act search warrant provisions are invalid.
I hope that you are all enjoying this narrative and fully understanding my ramblings on the matter. Perhaps, one day, I will get round to understanding the ramifications meself...:-) .