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.