If Japan says bitcoins can't be owned, do they mean japanese yens on a bank account can't be owned either?
That decision has been misunderstood. This was about how bankruptcy works when Bitcoins are involved. The court ruled that Bitcoins are fungible; that is, specific Bitcoins combined in a wallet lose their individuality. The question was whether people who deposited Bitcoins with Mt. Gox were entitled to get those specific Bitcoins back in the bankruptcy, if Mt. Gox still had them, or whether they were just creditors of Mt. Gox, in which case they get a proportional share of the liquidated assets.
To understand this better, consider renting a car to a company that goes bankrupt. You can get the entire car back. Now consider lending them money. You only get back a fraction of the money, in proportion to what's left.
There are cases between those extremes. If you stored wheat in a grain elevator, with the wheat stored in a common silo, and they went broke, that works like lending them money. Grain is considered fungible; quantities of it are interchangeable. Bitcoin is now being treated more like wheat than like cars.
This is only meaningful in bankruptcies.
