That's not entirely accurate most (if not all) projects these days do give out freecoins every now and then for marketing purposes...
Let me correct that for you. Most unlisted ICO, IEO, or whatever the newest project trend is give out free coins in the form of airdrops. You don't see established coins do that. Are there official Bitcoin, Ethereum, Litecoin, Cardano, Solana giveaways that we should know about?
and besides how is it scam if the airdrop is free aka no deposit is required to receive it ?
Airdrops and giveaways are no longer free. They require you to perform certain social media tasks. They are represented as free but they are more of a bounty.
An airdrop can have a malicious smart contract that steals your valuable tokens from your address. It could be pointing to an address where you are asked to perform a verification, download an app, and similar things. One trick that is being used is that a fake MetaMask pops up asking you to update your MetaMask. If you do, you install malware on your device.
Another way to trick the user is when he attempts to swap the airdropped tokens for some other asset. MetaMask pops up asking to confirm the transaction. What it actually does is it asks your permission to transfer a different valuable asset from your account. If you aren't careful and don't check and verify everything (and many people are too lazy to do that), you might give the required permissions and have your tokens stolen.
Not to mention that people are still falling for the "enter your seed phrase in this form for whatever reason" type of scams.