Airdrop is a some kind of marketing program for new coins. We have to some promotional activities like twitting/ Re-twitting on twitter, Sharing on facebook and messaging using telegram. For these tasks their offering us free coins as a payment for those activities. However ultimately those are not free coins, we are getting some kind of payment for marketing/ promoting works for their new coin or project.
You are reffering to what BOUNTY program is, not what airdrop is for twitter/facebook sharing things and such. An airdrop is like simple giveaway that often ICOs or maybe some alts use to gain exposure. It is often like singning up to their website, joining telegram group and be active, some will ask for personal details (where you can put false one, nobody is checking btw) etc. So like giveaway for simple tasks.
This is done to raise awarness more about an ICO and alts, since some of them even put affiliate links which you could share and this is great for them to gain exposure. Since they are the ones who creates those tokens they don't really have anything to lose since they are in control of distribution. I think that often you could get few dollars if an airdrop turns npt to be scam.
