You can use math to calculate the expected value of the user's balance, but it is still a possibility.

These kind of bonus should come with terms and conditions to avoid unethical actions from players (such as the mentioned loop). This is how the bonus algorithm should work, otherwise if your funds are remain the same while cancelling the bonus, everyone will claim the bonus and cancel whenever they lose the bonus, they can simply claim again and continue the loop afterward. To make it clear, you are not being forced to bet, your funds will remain in the balance if you did not wager(lose) it, for example, if you make a deposit, you claim the bonus and immediately cancel the bonus, you won't lose anything, or maybe if you make a few bets which resulting in lose, this is the only way that affecting your balance, which means the player funds are being use for wager before the bonus funds, and this should be mentioned in the terms and conditions to avoid any misleading. Spreading negativity will not help the case. Although it was the player fault for not reading anything before proceeding to click the cancel button. Duckdice has refund the player's deposit. The player should read and ask before performing those action, but he didn't. In this case, I can only see that the terms and conditions are not clear, which could misleading part of the players.