Wrongdoing isn't necessarily the same thing as "not fair," but in my eyes, it is fair because the user agreed to those terms of service upon registration, and their deposit was returned so they didn't actually lose any money
I want you to consider the two states of reality from before OP deposited to after OP was returned money.
Prior to deposit: No bets have been made. Balance is X.
After return: Bets have been made. Balance is X.
The fact that Sportsbet.io accepted bets means that they were willing to take the player's money in the case of a loss. Given that the player has instead won, the casino is now instead liable to pay up. Unfortunately, due to them using their arbitrary ToS, they are able to quote a clause that gives them absolute discretion and they are able to essentially nullify the player's wins.
Now imagine this.
I offer you a bet. If you win, I freeze your winnings and return your deposit. If you lose, I take your money. I cover details allowing me to arbitrarily freeze balances if I consider you a negative to my casino in a clause outlined within the ToS. You agreed to this. I can now act accordingly to everyone else who agrees to the ToS, because of course, contracts are all-encompassing and if you were too stupid to read it then you deserve to get scammed.