Forget about ethics for a while. Who should have ownership versus who does have the ownership differs. The protocol listens to the latter. And since ownership is defined by the protocol, whoever has the keys is the owner.
That just sounds like a poor excuse to take someone else's coins. You might not do it personally if you had the chance, but you seem to be justifying such actions. It doesn't matter what the protocol allows you to do, don't do it because it's the wrong thing to do. We are not robots. We are human begins capable of making the right or the wrong decisions. Taking or spending someone else's bitcoin isn't the right one no matter what network features allow you to do.
If I know your credit card number and the CVV number at the back, I have what I need on a protocol level to use your card for online purchases. Doing that makes me a thief though.
That's because there's a legal framework regarding car ownership.
Is there any legal equivalent about BTC?
You need a law to tell you what's right from wrong? You need a religious book or a preacher to understand what's the moral thing to do? Can't we figure out those things ourselves?