There are many projects that aren't scams (I would even say most aren't, it's just that most of the time the rewards are worth nothing). In most cases the amount is indicated from the beginning, it is just necessary to inquire before.

Concerning the airdrop, it's not free: in general it is necessary to do a minimum of action (tweet, facebook..), which makes advertising to the project (distributing crypto to many people makes advertising too)

On the other hand for AD who just ask for an email and an MEW address, I think the majority are scams