More info from Sophos' Naked Security and Graham Cluley:
New Android Marshmallow devices must have default encryption (mostly) | Naked Security
Google mandates full-disk encryption on new Android 6.0 devices
New Android Marshmallow devices must have default encryption (mostly) | Naked Security
Google mandates full-disk encryption on new Android 6.0 devices
Google insists on full encryption for Marshmallow
