Until iCloud/iMessage, all the actual information was transmitted through third party services (i.e. network providers, email services, etc.)

Why go after the myriad of handset manufacturers when you could just get the network providers on board?

There are things network providers can't do: activate mic remotely, capture local-only data, keylog apps that use encryption, etc.

The list was about joining PRISM, it doesn't say anything about backdoors in mobile phones. They may very well be present in all iPhone generations.