When sending a message to a member via the Directory the default From address for a Moderator is group+owner@... If the Moderator is not set to receive owner emails, they should not (by default at least) have the option to send emails from the owner address.
I generally support the idea that the drop-down menu should omit the +owner address if Owner Email is currently set to None.
Two clarifications:
The +owner email setting is a preference, not a permission. The same Moderator can, at any time, click on Subscription on the left-side menu bar and turn his +owner emails back on.
The same thing can happen when someone sends a message via the Member List. If this issue is really an issue, the remedy needs to be applied in both locations.