I suspect the reason they are releasing the keys is to give themselves plausible deniability. This way they can still go ahead with the attack and just claim it was greedy bitcoiners scrambling over the coins.
Possibly but I think instead they are just using greedy Bitcoiners to execute the attack for them. These private keys had Bitcoins that when spent cause very spammy transactions that can clog up the network. It's obvious their intention is to attack the network so what they are doing is still probably illegal.