Yes, some airdrops can be a scam in a way that they might claim to give you some free coins that they will be distributing it after their token sale completion for joining their telegram group, following on twitter so that their community grows and they can attract more legit investors to buy their coins & in the end the ICO makes an exit scam without distributing airdrop coins and coins to the investors. Also, everyone should avoid those airdrop coins that require KYC, don't sell your information for just a few tokens that have a value of just ~5$ it could be nearer to scam.
Scam means dishonesty or fraud. That is why people are calling some airdrop as a scam because they didnt give the rewards as they promised even it is free. The bottomline is we are promised to be rewarded after signing up but they didnt give back anything, we already call that as scam or fraud.