cashback calculated before bonuses removed makes no sense

why are sites still giving cashback on the full deposit even if you blow half of it on some goofy slot bonus before you hit the tables? seems like a messy way to do it. if i deposit for blackjack, take a bonus, then lose half grinding through a 30x slot playthrough just to qualify, the cashback is calculated on the original deposit, not what’s left when i hit the real games. how’s that supposed to be fair? am i missing some angle here or is this just one more reason to steer clear?