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.
Casinos do have tools to detect gambler information from IP address, device information used for login and betting, betting pattern and more so that they are able to detect suspicious cheating accounts. Sometimes they don't need KYC information to conclude multi-accounts or cheating.
I think for promotion and small bonus events, KYC is not necessary to conclude cheating cases. KYC is only helpful for complicated cases with too big capital and casino need to have it to handle cases very confidently to avoid exploitation on their budget as well avoid unnecessary damage on their reputation by wrong initial handle.
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.
It's true to read rules of events like free spins before joining it. If you see rules are not attractively good enough or contains some requirements you don't like because you see it violates your privacy, simply ignore those events.