This principle does exist in the real world, and maybe it would be of benefit to explain it.
In England, America, and under other legal systems derived from English common law, this offense is known as "conversion." It is distinct from theft, wherein one actually takes the good knowing that it belongs to someone else, or receiving stolen property (i.e., accepting something from another that is known to be stolen).
Conversion is a tort, which means that one person has been unjustly enriched at another's expense, and the damaged party is entitled to sue. It isn't a crime, so the damaged party won't get help from his friendly neighborhood prosecutor, there is no jail time, and the offending party only has to return the item.
Moreover, the same person can be tried criminally for theft, and civilly for conversion, and even win one case and lose the other. A poor man stealing a few coins or a loaf of bread could, conceivably, be found not guilty of theft, but he could be forced to return what he took.
Without such a provision in law, a victim of theft would have no way of recovering something that was stolen from him and sold to a third party.
With that said, GEAS's legal system is more akin to the civil law systems found in just about every country that wasn't once part of the British Empire. Under these legal systems there is something called a "delict", which is sort of a generic term used to describe some violation of another person's rights, and a "quasi-delict", where the violation is unintentional. There is no specific "tort of conversion," but you're still violating someone else's rights when you take something that belongs to someone else. I'm honestly not aware of any previous effort to punish behavior like this, but I find it RP-consistent.
I could see more of a "spoils of battle" approach taken in Arborea, where the victor is entitled to the possessions of his slain opponent. I don't know how this would work in a world where fallen opponents occasionally are resurrected, but someone else can sort this out.
ICly, Furius stands ready and willing to discuss legal theory with just about anyone.