I stopped joining airdrops long time ago, and surely some would have done the same as well.

It's quite good idea for a project to do the same as what you mentioned in the OP, since it is indeed vulnerable to abuse. People making an alts here and there with their fake information. I wouldn't term that "penalizing" though. I would describe it as limiting the users involved like what tsaroz mentioned "lucky draw". It is not 100% foolproof, but it surely reduces abuse. I believe some airdrop "hunters" dump the coin anyway, so what if the platform just decided to limit it. There would be lesser number of people dumping their coins, though I know that even common airdrops aren't that much of a percentage out of the total supply.

Will it do any harm to crypto? No. I don't think that attention to crypto is highly focused on airdrops anyway. Many airdrops turned out to be a scam long time ago and if that hurt crypto, there wouldn't be that much of newer projects coming right out these days already.