Any e-mail address that signs up is immediately added to the simplenews_subscribers table as an activated user. Users are only deactivated if they do so (or are made so by an admin) specifically; this is what prevents people from being reactivated.
The simplenews_snid_tid table keeps track of what newsletters people are signed up to, where snid is the Simplenews subscriber ID and tid is the newsletter taxonomy ID.
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