Ofc the ones to blame are the fraudsters. No way in hell is there a world where the act of scamming/taking advantage of a loophole is the right thing to do.
For me scamming is very different from taking advantage of casino promotion. I'll make an analogy for this to explain more properly:
Taking advantage in sense of participating in it is very different from exploiting the glitch or loophole of a casino. It may not be cheating in its own meaning but part of the gaming community see this as cheating because of the unfair advantage it gives to a player.
Scamming is like cheating, it's like using a 3rd party-tool to a particular casino or website to rigged their game for your own benefit. It's stated on their TOS that using those are illegal and can face some serious charges if proven to be guilty.
Mostly, rule about a promotional exploit is not listed on the TOS but promotions have their own set of rules and the moment a person participated in it, they are bound to the said conditions.
While promotion abuse or exploit doesn't violate anything on the TOS making people not afraid to do it. It is the fault of casino owners because they did not review properly if there will be a loophole on their promotion.
It does violate the TOS, if a Casino implements a 1 account per person policy, sometimes 1 account per IP policy, then an exploit through creating multiple accounts to abuse the bonus is breaching the Casino TOS. So in this scenario, the owner created an anti-exploit condition but abuser just ignore them.