Their cashback bonus sounds similar to bitcoinrush's cashback instead of free bets as the reward you'll receive smaller amounts of bitcoins in your balance.
Also for other that wants to know more about the bonus can you tell us what's the minimum free bet amount?
This isn't a terribly attractive offer, to be honest. You are not creating an incentive for the user to stay and use your website in a way that is beneficial to the user. But just extending the engagement time per user by a fraction and calling it a 'Bonus offer'.
Some sportsbook that I use doesn't have incentive, them having a cash back is nice but I wouldn't use them due to kyc on registration.