Regardless of who benefits between the two then I can only say that they are just part of the industry that serves your needs. Nothing is free and both use their customers to make a profit - it's worth it for what and who they serve, right?
You need the casino to serve you to gamble - so you have to pay them instead of just betting your money. The same thing about brokers - you have to pay trading fees [or whatever] if you use the facilities they provide and it is all a win to win situation between customers and industry players. There is no blackmail because you have all agreed to the terms of service - if you say it's blackmail then re-read the conditions for using their services.
That's exactly the point, we know that they are earning money from us, but we are the ones who are using the services they are providing, so we can't really say that they are exploiting us or using us for their own benefit because that's not entirely true. If you are a crypto day trader and you use a centralized exchange for your trades, you are getting to use the tools and services provided by them and that is what they are charging you for.
Similarly, when you are gambling on a platform, you are basically getting the opportunity to gamble and they are the ones providing you the opportunity, so if they charge you a percentage of your wins just as a fee for their services, there is nothing wrong in that and they truly deserve that for what they are doing for you.
So, both brokers and gambling houses are only getting what they are working for just like any other business in the world.