I'm confused. Why are decryption keys in NVRAM? That seems to negate the purpose of at-rest encryption if you can retrieve keys from the device even after shutdown.

Well they're encrypting an SD card, so this still defends against its being removed from the camera and stolen or left in a bar or something.

But honestly from the rest of the story it sounds like the camera manufacturer was selling their pressure housing moreso than the off-the-shelf camera hardware inside, and was not particularly concerned with whether/how the storage was encrypted.