iMessage is definitely "backdoor-friendly" due to how the CA server works. In past court cases (e.g. [1]) Apple has said they do not have the capability to wiretap iMessage and they have refused to create it, and the government has backed off. Maybe in part because in those cases there were active iCloud backups which provided messages in near-enough-to-realtime that the government wasn't in a good position to force Apple to hand over access to the backdoor. Or maybe because the targets of those particular investigations weren't unsympathetic enough. This case is the one the government has been "waiting for" to push the envelope and see if they can force Apple to provide a backdoor against their will.

[1] - http://www.nytimes.com/2015/09/08/us/politics/apple-and-othe...