The casino in this scenario is taking a rather small and calculated risk, hell - they are simply diversifying their many advertising streams, one of which will be to use streamers to promote their services. In many cases these streamers are very valuable to them, as they have a much more personal connection to their audience, who might be more willing to participate in activities that are promoted by this semi-celebrity in their eyes. The fact that they are not always labelled as receiving paid endorsements (even if this is illegal in their jurisdiction) is another bonus to casinos & sportbooks. There are some casinos out there who do not use these advertising methods, but you'll find they are eventually pushed out by the bigger ones who use these strategies, even if it seems unfair.