If there were ownership in bitcoins, when you received bitcoins from another person, you would be exposed to the risk of someone demanding that you return the bitcoins to them, arguing that they are the owner instead of you – possible reasons for this could be, for example, if the bitcoins were stolen from them or if the agreement under which they sold the bitcoins was nullified.
http://www.coindesk.com/what-tokyos-mt-gox-ruling-means-for-bitcoin-in-japan/Interesting read, easy enough to follow even for someone without a legal background.
It seems like a few issues brought up would be exchanges NEED to have some sort of cold storage offline procedure to protect hacks.
Also he's basically saying btc is fine to use under Japanese law, bit don't trust and exchanges because it kind of creates a "joint ownership" and if the coins get lost, then you're out of luck