I believe most times casinos intentionally do that upon launch to allow traffic to flow in and when they start getting the traffic they desire, they update their terms of service and registered members would not know that until they want to make a withdrawal and the casino requests for kyc and then they would tell them that their country is on the restricted list and would get their account banned or suspended after they had deposited funds to play and won games.
If they do this, they play ugly cards against their customers and this approach only harms their business reputation. KYC if is implemented mandatory or stricter than before, must be announced publicly on their platforms. In addition, the company must give their customers a kind of window time to withdraw fund if they don't want to comply with new and stricter KYC policy on the platforms.
It's better strategies from policies and media announcements from centralized exchanges to online gambling sites. If you don't play ugly cards but have bad approach somehow, your business reputation will be affected more or less, but generally it's not good.
I have seen many complaints here in the past from members about kyc issues and some ToS that weren't available when they got registered with the casino and they felt since it was not there when they registered, they have nothing to worry about but the funny thing is that some casinos always have a clause attached that amendment to their ToS is at their sole right and do enforce it whether their members are aware or not as they have the sole right to make changes at any time. All this they do, they do not inform or send notice to their members. It is always at the point of withdrawal that is when the issues begin and members would be blamed for not reading the ToS regularly before further engagement.
This is mostly done by new casinos looking for traffic and when you ask them about this aspect of their ToS, they say that for the time being if there is any suspicion on a players account, that is when a kyc is likely needed without clearly stating it if they conduct kyc for registered member before they could gain access or make a deposit into their dashboard with the casino.
This is just the issue with new casinos looking for traffic, they just possibly intentionally do that to get the traffic they want before they implement the kyc features openly as they know many people do not fancy it. They forget that this also affects the reputation of their casino and paints a bad picture of them whenever the name of their casino is mentioned.