Whenever something is given free, we need to know on what basis it been given. For example, something is given as a compliment on some purchase it doesn't mean it is free. Indirectly we'll pay for it. Here the consumers are happy about the compliment while the seller is happy with added revenue. With bitcoin giveaway same is happening, the scammers just to make people happy term it a giveaway and the common users fall for it. Finally, only the scammer feels happy. 
Too good to be true. Unfortunately, many people are tempted by this bait, moreover, we are all in a difficult situation, so when there is a tantalizing offer it will be immediately taken without thinking of the risks.I found a lot of invitations to invest with the lure of thousands of dollars in a wallet, it is suspicious, even if there are people who can get it the easy way, surely he will not be as easy as that revealing his secrets and inviting others to follow the trail, it's a trap. As you explained, we must analyze the background of the reward, check on the official account of the project or company listed (if any). And the most important thing is not to trust anything easily, especially if the investment offer or airdrop requires you to send some funds, it's a scam.