Wrong. Do you think casinos are stupid enough to allow players to easily gain an edge over them? Think again. These promotions benefit the house itself and they never lose the edge in the long-term.
Few players get lucky and win big thanks to these promotions in the short-term. The only way players could gain an edge over gambling sites is through unique strategies(Matched betting, Roulette wheel bias etc).
In the end the casinos always win, even if players win 1 million $ on some slots. Players that try to defeat the casinos with performing matched betting (i.e. surebets) can see their account locked as in many TOS, that kind of betting is prohibited.
Some bonuses before are being stopped because of this greedy action, I guess those are not the real gamblers instead those who are trying to get easy money from bonuses and will withdraw it later on once they succeeded. Roobet is one of the reputable gambling site today, I'm sure they already know what to do if there's an abuse.