I don't want to be the bearer of bad news, but I think this is clearly stated in their TOS.
Please don't shoot the messenger!
11. Winnings from wagers placed using a free bet are calculated as the total returns less the amount staked (including the amount of the free bet staked). The value of your free bet is therefore not included in, nor cannot be withdrawn as part of, any winnings you make from your free bet.
And this has been the standard for handling free bets for like forever. What you get at the end is the total amount minus the stake.