I agree with everything else you said here, but UNETHICAL? Theft, extortion and deception are considered to be breaking even the Non Aggression Principle. Which is... The very basis of libertarianism. By extention of that very notion, simply remaining idle while you see someone being scammed and knowing it could also be considered unethical.

Given that "not your keys, not your bitcoins" is true, then Coinbase would not only be liable of allowing the extortion to take place while they knew so, but it'd also be against their rational interest since it was as you say, their money. So I don't see why preventing someone from a sure monetary loss would be unethical by any standard.

Sure Coinbase has other unethical practice. Maybe their mere existence as a wallet is counter-intuitive for BTC, but preventing theft isn't unethical. Unless we bitcoiners want to build a new book of ethics ofc.