you should already know that there is no such thing as "free" in the world. so things such as "airdrop" has never been actually free even though it may look like it at first!
what they do is that they are actually "using" you to make money. you are working for them by spreading the word and freely advertising while getting some useless token they created out of thin air and they call that "Free".
these token creators want to make more money. and to do that they usually use KYC to pretend they are legitimate so that more people are fooled and invest in their token so that they can make more money. sometimes they also use KYC as an excuse to get your private information and then sell it on the dark market for more money!