It's going to be the norm of being asked for KYC I believed, so you should be ready when one casino will ask you to do that. I guess it's the question of practice though, as the OP mentioned, it's only when the bettors won big that suddenly the casino will ask you to submit info. And when you comply, still, there are suspicious and then you might not get your winnings, hence there is a scam accusations against the casino itself. So it's better to ask for KYC in the beginning for me, and not get gamblers info false idea that they won't be getting KYC'ed.
Unfortunately, so many casinos are not doing this, they allow people to join and do nothing while betting, there are people who already wagered 5 digits without being asked to do KYC, then they asked when the players are about to cashout huge amounts of money that's where the issue for the player start, besides the long wait they are in agony if the casino will release their earnings.
The scam section is full of these players wagered 5 to 6 digits with no KYC then the casino asks for KYC when the player wins a big amount only to be accused of cheating or having multi-account.