far as gambling is legal, I do not see any problem with celebrities or any other people promoting gambling sites. But all I have to say is that celebrities should always try to verify any site they are given to promote, whether it is a gambling site or any other site, to know that it is legal. So, if any celebrity can follow the right steps and laws that guide any promotion, there will be no problem in the future. Therefore, celebrities should not be convinced by big funds to promote something that is illegal.
Usually they use terms that won't put them on blame e.g. "I, my name, not affiliate nor endorse the brand in my photo/video etc". Depends on how viral are their content, if their content went viral, they can't use that terms to make them right because the government will caught them in order to make the citizen happy.
Celebrities are people who have luxury lifestyle because they need that in order to make them "expensive" hence they're promoting illegal casinos or even fall to prostitution.