There are multiple causes for this ...

1. Some legit ICOs can use KYC to reduce the number of fake accounts trying to benefit their "generosity".
2. Some not-sure-if-legit-probably-no will sell the KYC info to the highest bidder.
3. Some bounty managers may use KYC to not pay for some work done.

All in all, since the ICO/bounty/airdrop is made my people you don't know and you are not sure about their intentions, it's safer to not give out your info so easy, since you may have nasty surprises later on.
Even if you'll lose a couple of easy bucks because of that.