I've seen people calling such airdrop as paid airdrop. Team behind it usually claim it's to prevent cheater and bot. But i don't like this kind of airdrop since,
1. If amount of demanded TON is too low, it won't stop dedicated cheater and well-programmed bots.
2. If amount of demanded TON is too high, people will perceive it as attempt to trading their TON with token that could be worthless.