To add to andris's great response - the opposite is also true - even if the system does not have encryption turned on, if the drives are encryptable, you can't add non-encryptable drives to it either.
While these two scenarios are sometimes inconvenient, it is required for compliance reasons and to prevent data leakage on encrypted systems by many of our customers.
We have an option in ONTAP 9.1 of NVE - NetApp Volume Encryption - which does software encryption of selected volumes only.