For promotions that are quite interesting and profitable, casinos usually ask for KYC or verify their data for customers who want to claim bonuses.
This is one of the strategies of the casino to still get KYC from every customer but in a way that is more subtle and doesn't seem obligatory because I'm sure casinos also realize that most gamblers don't like KYC.
You're seems like saying KYC casino is taking advantage to force their user to submit KYC when the gambler joined a promotion, but that's a way for the casino to know if the participants weren't using multiple accounts because it's not fair to the other gamblers.
Simple, if you're worried about KYC, just gamble with small amount money and don't join any promotion or free money/spin, the casino wouldn't ask you to submit KYC since your account have normal activities.