If the creator did not legally relinquish ownership (e.g., via a smart contract or binding terms), brute-forcing the key might still be considered theft.
If the owner did not clearly relinquish ownership (e.g., by publishing a signed transaction or legal agreement), taking funds may be unlawful.
Intent: If the puzzle is structured as a brute-force attack rather than a solvable riddle, it could be argued that the creator is inducing illegal activity.
This is impossible to solve, man. Not sure what's the fuss about? You want to arrest the creator of this puzzle? Aren't you? Or the participants? Come on.

It’s mad hard to prove someone’s in the game. Unless they’re buggin’ and move all their BTC where there’s KYC. Like Binance or somethin’. Me? I’d probably use an atomic swap and Monero. But even if they catch a trace, they’d need a whole army to roll up here. A whole army. Maybe even a bunker buster. And even then, they risk startin’ World War III with another country.