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 have been scamming their customers for years and the casino still finds a way to gain new customers somehow.