Surely if the device automatically decrypts data as you read from it without having to supply a key or passcode of some sort then you're not protecting it again

Surely if the device automatically decrypts data as you read from it without having to supply a key or passcode of some sort then you're not protecting it against anything! It may automatically manage/store the actual keys used to encrypt the data, but there must be some mechanism to ensure decryption is not permitted until you supply some external form of credential (which ideally is cryptographically required to access the keys - if not then obviously there's a risk the device's controller code could be reverse engineered to bypass the credential check).