Airdrops are expensive and costly on the team side.

We recently did an airdrop for ravelous and sent out coins to over 9400+ users.

There were thousands of duplicates/people using other peoples info that we had to parse through. (~11k applications had been filled)
Lastly, The cost per TX on ethereum for sending out coins adds up.

We created a custom smart contract to send coins out to 80 users at a time saving us ALOT of money on gas per transaction.
Even so, we had to run the airdrop deliveries in batches of 1 gwei (at night so that they would go through the network) and the process took days and cost us a couple hundred dollars.

If we had tried to run this same process at a higher gas price (2 gwei or more) or tried to send them out one by one to each user we it would have costs us a fortune for this "Free airdrop".