for something to becomes yours you need to have 100% control over it once it's in your hands, bitcoin, cash, cake, an orange etc... are item that can't be taken back unless you give it back or is taken from you by force.
If you want to look it like that than you don't own anything at all, and your bank account with all the money is not yours, but you would still accuse bank if they first send you virtual money and than they delete it from your account (owned by them).
Or maybe you are so liberal that you would just ignore it and say it was never yours to begin with.
You also don't own your bitcointalk account, but you would still complain if someone hacks and take it over, even if that was theymos.