Yess you are right that mate and The most known tricks for those scam casinos are they are offering small amount of bets and then you have a chance to win big amounts without asking anything, then after you won then they will have new rules which is what you said above once a gambler want to withdraw his/her winning then the casino make a thing that a gambler can not make his withdrawal. Or worst they will abandon the request from the gambler that he want to withdraw.
Most of the time when a casino is a scam and they do not want to pay they will argue that you violated their TOS in some way or form, and while you could be completely sure this was not the case they will never budge, as doing so will mean they are basically admitting they tried to scam you and now they are backtracking.
However while common sense should dictate that a casino like that will not last long, we have at least one example of a casino which has been scamming their customers for years and the casino still find ways to gain new customers somehow.
And that is a sad reality. Most people are not yet aware of these scams yet. They still fall for some sort of marketing gimmicks along with the online casino's bonuses that are often too obvious to be a scam but people still does catch those. Casino's like giving a bonus such as added hit rate when they deposit $100 dollars. Or just as mentioned b the OP, the casino's purposefully give you difficulty on withdrawing your funds until you gave up asking for it. Some closes some do not. Most reopens just with another name.