Email announcement is adding in Registered Users and will not allows us to remove that group.

We created an email announcement to be sent to a specific user group only and then saved (pic 1). We edited the email announcement and saw that the Registered Users group had been automatically added to it (pic2). We removed the Registered Users group and saved again. We edited the email announcement again, and the Registered Users group is back.  We cannot get rid of it. We want to create an email to go to a specific customer group only, but we can't because of this. Is this a bug or is this working as designed? Thanks for your help.