A newsletter indentifies its recipient in two ways: all the users directly assiciated and all the users with at least one of the roles set on the specific newsletter configuration.
Usually a user with a role is not only a subscriber but it is an active portal user, may be manually added by the administrator. So the user setting about the mail he want or not want to receive are intrerpreted in different ways.
How a newsletter loads and discard users?
The newsletter system first loads all users that match the configured roles and discard the not active users but the "do not send newsletter" setting is ignored.
For compatibility, if a user is anonymous and his profile report "do not send newsletter", he will be discarded.
All users directly associated are loaded: users already present in the previous list are discarded to avoid duplication.
Disabled users or user with "do not send newsletter" set are discarded.
Finally the resultant list is processed.
Why the "do not send newsletter" is ignored?
Users who can log-in the portal are not simple subscribers and the Newsletter system can be used to send spot emails to communicated with them.
This is why the system distiguishes between "subscribers" and "regular" users and gives a different means to messages sent to "roles" or to "newsletter-associated" users.