Yes, even free airdropped coins can definitely be a scam.
For example, imagine someone comes out with "Heaven Coin" a coin that promises that when you get enough coins to create a "Heaven node" you get to go to heaven and meet Jesus. They airdrop a bunch of coins and people start trading.
Maybe you didn't invest anything, but you trade your Heaven Coins on an exchange. Maybe you even tweet about it.
Now you are part of the scam. You created volume on the exchange and made it look like a legitimate project. Even if you didn't sell anything, you helped create the appearance of a decentralized project - which WILL suck in some foolish investors. You are part of the problem.
It is important morally to avoid scams. We are all in this world together and just because someone is a fool doesn't mean we all don't lose when someone steals that fool's money.