How is this different from the current PoS systems like for Blackcoin or Mintcoin when it goes 100% PoS? People generate coins through "minting" instead of "mining" By keeping coins in your wallet, you get coins generated randomly as interest. It keeps the incentive to hold onto the coins and the limited supply keeps the coins valuable. No need to spend any power on hashing to generate coins.
it's the incentive to hold a given number of coins per wallet at given times. doesn't matter how fresh the funds are, only the fact you held them at a given time is important. The blockchain provides the proof you held them, and your private keys qualify you for the rewards. Technically it can be implemented through a transaction the wallet initiates after the airdrop (which other nodes would validate against the block chain). That it's a per-wallet reward reflects the idea that anyone is eligible (and we're not checking if you hold multiple wallets because that is technically infeasible, so wallet hoarding is legitimate).
Current PoS solutions requires funds to be held for a minimum time, after which the PoS is generated at random times (for Yacoin at least), and typically the PoS is low (in the order of percents).
This airdrop is at fixed times and the reward greatly exceeds the held funds.
Christan