They opted to invite some they felt would like that sandbox better. At they same time, they decided not to invite those who previously were destroying their sand castles they had built.
It would actually have been pretty counterproductive to do otherwise. What would be the point? The issue was creating a new community among those who shared certain views, so they provided extra incentives to only those they thought might possibly share those views. Only if they knew from previous actions the parties DID NOT support the ideas behind the genesis of the new chain did they have no reason to give them incentives.
Furthermore, a previous STEEM fork called Whaleshares gave no incentives to those holding STEEM. They based their airdrop on holders of BTS who submitted their BTS address in time. There is no right/wrong way to decide to give stuff away!
If there is, I've got a bone to pick with US tax law for incentivizing marriage with all kinds of tax breaks when I choose to be single. Or how about business tax deductions when some people choose not to start businesses? Child credits for some when others choose not to have children? I mean, if we're going to say everyone has to get the same rewards regardless of whether they engage in the actions we want to incentivize, why stop at airdrops? There are more important things.