Airdrops were so popular among the crypto-community over the years. One of the key features of the airdrops is, it gives every participant a chance to get some free tokens by completing few tasks. Most importantly it is totally free of charge. But nowadays we frequently see a trend where the organizers of some of the airdrops ask to send a small fee prior to sending the tokens. In some cases, they put a condition saying that in order to withdraw tokens or any fund received from the sale of tokens, it is mandatory to take part in their ICOs. I would rather call such events as "Scam-drops" as they not only dilute the meaning of airdrops, but also make people to move away from future events due to lack of trust.
That is the absolute truth, airdrop participants have no business with ICO unless they want to, but putting that as a condition means something is wrong with that platform and hence that platform or project's airdrop should be avoided because the team will not pay but rather are looking for a way to attract more investors and possibly exit scam. Also, another way to point out fake airdrops is, any airdrop asking for transfer fee is probably scam, that is, how can participants pay before you pay them their rewards and most times, airdrops like that, ends up running away with the transfer fees collected.
Nevertheless, out of all these, there are still good airdrops and when one sees them, their condition are clear and not hidden thus possessing all the features of an airdrop, which is, do the given tasks and get paid within a stipulated time; but they are just few.