This is the kind of reasoning used by those who say, "I'm not a thief for stealing his wallet; it's not my fault that the fool fell asleep with his wallet in his hand"
Obviously, it's unethical and immoral.
It's like breaking into a friend's house, seeing his seed, and stealing it, then saying he should have been more careful.
Now that it has no legal implications, that is something else.
Satoshi himself kindly disagrees. Whoever has the key, owns the funds. Now, my friend should just use private key 0x01 and transfer his life savings into that address, right? No one will ever touch it.
Last time I checked, it wasn't unethical or illegal to add two numbers together.
What's next, unethical to use quantum computers to break all cryptocurrencies? Seriously? You'd need to have a word with China about how non-ethical it is to break RSA first, since they just did that, before getting slammed by all big tech giants following along.
Take the coins from someone you know and let them know it was you. See if the law says the coins belong to the person with the key. You'll probably be arrested, as is normal for a crime.
What Satoshi said is a philosophical and technical approach to the idea that anyone with your keys can spend them, because that's actually the case. But that doesn't mean it doesn't have ethical and legal implications.