The vast majority of casinos have a list of "restricted" countries. This means that players from these countries are not allowed to open an account and play at the online casino.
Many casinos only state the restricted countries in their Terms and Conditions (T&Cs), so it's the responsibility of players to check whether they are allowed to play at them or not.
Some casinos allow anyone from a restricted country to register and create an account, but only notify them about the restriction when players attempt to make a withdrawal. As long as a player is losing, the casino agrees, but as soon as the player wins and wants to withdraw money, it uses the 'restricted countries' rule to refuse the withdrawal and block the player's account.
This is, of course, extremely unfair, as players might register without knowing about the restrictions, and casinos are taking advantage of their mistake, often knowingly and intentionally.
If a player is able to access the casino's website, open the registration form, create an account, deposit and play, it's understandable that they assume that they are allowed to play at this casino. Some experienced or careful players might read the T&Cs and find out that their country is restricted, but these players are definitely in the minority.
It is not acceptable to let players gamble if a casino knows that they are from a restricted country and if the casino plans to refer to the rule about restricted countries whenever a player requests their first withdrawal. This is completely against the rules of fair play, as the casino is knowingly letting a player wager money without a chance to actually win something in return.
The right thing to do is to check for restricted countries during the account creation process, and to not allow players from restricted countries to register and play. If the casino knows about restricted countries and also knows the player's country of residence, it's not a big issue to enforce this rule.
If the casino allows a player from a restricted country to play because it hasn't implemented this check, it should accept that it was the casino's mistake and pay out all winnings to the player despite the fact that they shouldn't have been allowed to play in the first place.