All your data are encrypted using your master password whereas your master password itself is encrypted with a random generated key and stored in application’s private storage.

In a non-rooted device, application’s private storage is well protected by Android security model, however in rooted device any root app can access private data.

That’s the reason, Authenticator Plus enforces PIN lock in rooted device and your master password is encrypted using your PIN for additional security.

How can I disable PIN Lock?

It can be disabled if your phone/tablet support Hardware backed key, otherwise it cannot be disabled.