I don't think it is a bad thing advertise gambling, as long it is obvious one is being paid to do such advertisement. We are talking about a formal sponsorship with an entity and the person doing the streaming, so there must be some legal grounds for it. If the people watching the stream are not disclosed of the sponsorship and purposely are made believe there is no relation between the streamer and the casino, then that would be misleading and probably illegal on a handful of jurisdictions.
Keeping those points aside, I don't have any personal problem with it, in the end each one of us has a free will on what to do with our money.
We could also talk about cigarette vendors and they fact they know they product harms people's health... there are warnings in the package, there are advised by doctors not to smoke and the final choice is of the person to whether buy those cigarettes or not. It is about the same with gambling. The key word is responsibility.