I know there may be many people who are not playing at duelbits actively. By announcing the giveaways only on Twitter means that they want more inactive or new people to join their site.
This is exactly the points of promotions which is to reach out for the mass potential customers that havent registered and try the site. There is really no reason to keep a customer staying by giving all those free stuff. In the online gambling business, as long as the site never delay payment and it gives decent amount of rakeback especially to those that wagers alot then people are staying
Duelbits can also take advantage of its emailing services by giving notifications to players about their Twitter code giveaway. If I am not mistaken, other gambling casinos even give free credits and free spins to encourage their inactive players to return. I don't know if Duelbits had tried this kind of promotion in order to entice their inactive player to be active again.
It is an SOP and an urgent matter to be prompt in processing withdrawal and I do not think there will be an issue about rakeback since a player can claim it anytime.