Retweet only promotable users are setup and give consent to have their tweet content promoted through the advertiser's account manager. So it's basically manual whitelisting by Twitter sales team or your partner manager contact.
Add some extra info about multi-user-login issues:
Promotable users come in two types: FULL and RETWEET_ONLY. A given advertiser account can only have a single FULL promotable user and that's the owner of the account, but can have numerous RETWEET_ONLY promotable users.