Can I ask what the harm is in joining the airdrops? As long as you're not buying into the "scam". Not asking to incite any anger just simply asking of interest.
I can see a couple of possibilities. When you sign up for airdrops they may ask you for personal info(name, email, various usernames, ip address take you to a spoof website etc.) With this information they might have enough information(depending on what you give them) to steal and use your Identity for one purpose or another.
Another reason not to support airdrops you may think is a scam is by becoming a part of an airdrop you validate it. Other people see you signed up and think, hey so and so joined it must be legitimate. Its very possible that they ask you to help bring in others who may succumb to the scam.