I think it's a common policy for casinos not to allow withdrawing money won with a faucet. Or at least to have strict wagering requirements, so that this won money is used again and again for other bets and then a person can withdraw whatever's still left. I couldn't find such rules on their website, though. Bitdouble has positive trust on the forum, but the account hasn't been active since summer, so I'm not sure you'll get any more explanations from them on the forum, unfortunately.
In any case, you didn't really lose anything, you just weren't able to withdraw what you won with free money, so it's not that bad IMO.