It is scam because they offer you free coins or tokens for signing up and sometimes tweets (airdrop tasks) but finally you dont receive anything, because it was only for collecting data. So this is lie from the start, they can also steal your tokens If you have them a lot.
It was not only for collecting data, but for free marketing as well. Airdrop hunters seem to do so little for a project, but in reality they make th biggest part of followers count, which really helps impress the potential investors.