From what I'm reading in other places this seems more like an oversight than malicious intent. It was present in iOS 7, then broke in later versions. The e-mails themselves are still fully encrypted, and the iPhone data itself can be encrpyted, the main trouble seems to be what's transmitted during sync, since the actual researcher[1] claims a valid workaround is simply disabling mail synchronization.

1. http://www.andreas-kurtz.de/2014/04/what-apple-missed-to-fix...