One of the reasons why gambling is called a scam is that most websites cheat gamblers in the way you have discussed. There are also some sites that deliberately do not let gamblers win. Maybe the developers of those sites have not prepared the games/casinos in a user-friendly way that why gamblers lose most of the time.
Another reason why gambling is called a scam is that gamblers mostly lose from reputable casinos or gaming sites as well as betting sites. You can be confident in getting your money back from reputable betting sites but some others are unreliable. I consider the trustworthy side when betting which is confident in getting money.
It is not gambling scam them but the site scam them. That is what they must understand. They must know that gambling will not let them win so casino will take all gamblers money without any chance to recover their losses. If they don't want to scam or "lose their money" in gambling, they don't have playing gambling instead save their money and use it for other things.
Till now some people have never understand the difference between gambling is a scam and a website that is being registered in the name of gambling is a scam, gambling can be only be scam through the mindset of the owner of such a gambling and if the particular gambling website scammed people that does not mean that all gambling website especially casino is a scam platform but people does not understand it in that way but they only believe that provided that one particular gambling website refuses to make the win in gambling automatically that particular gambling is a scam and they would like to generalize it to other gambling website that all gambling is a scam without any proof, not all gambling Websites is a scam.