I guess these are sort of marketing strategy which is part of their promotions that shows highlight only of the percentage not the totally terms of it so people get easily courage to play and deposit it seems that you already identified the event works. Still i consider that the deposit bonus is different from the cashback because in deposit bonus you will be credited upon deposit and then the cash back will be credited during the progress of the game lets say cashback = total deposit/wager requirement.
Usually, cashback is different from the deposit bonus. That is true, you will only accumulate your cashback depending on how much you wagered for certain session/s. However, the percentage varies from one casino to another or if they are having this feature in their casino. Thus, need to check their features and don't expect that every casino has this perks.
Deposit bonuses and cashback are two different things but in essence both give gamblers bonuses but with different scenarios, and another thing of course reading ToS is an obligation for a gambler so that they avoid unnecessary mistakes, you have done it unknowingly but it's good that you realize that it is your own fault.
So maybe I can say that the unwanted thing happened because you were not careful in responding to the bonus so that you took the wrong action, every time I gamble I often find such bonus offers especially in some types of local online casinos in my country which even offer a 100% deposit bonus, it looks tempting because that way you can play longer and maybe with a large balance you will be a little closer to victory, but one of the reasons why I prefer to ignore it is because of the many requirements that seem unreasonable or impossible to pass, you should know that in the end the promotional scenario will only benefit the casino.
Every reward/perks offered by the casino has certain conditions to be met. So don't rush yourself depositing to the site because you have seen such promotional offer. Always check the terms attached to it, if you think it is achievable, then, why not? But if not, better let go such offer because more than likely, you will just lose all your bankroll before you complete the requirements for such reward.