Non-public groups appear on contribution receipt

My online contribution form uses a profile that includes a "groups" field, which allows people to sign up for our mailing list. Only groups visible on "public pages" are shown as options. This is correct.
However, when the contribution receipt is sent, that field lists not just the public "mailing list" groups, but every group that contact is in. In other words, it does not respect the setting to have a group be only visible to user admin.
So if you have privately put someone in the "Assholes" group, this could be very embarrassing.