Oregon State's definition of "Thief":
64.005 Definitions. As used in chapter 743, Oregon Laws 1971, unless the context requires otherwise:
(1) Appropriate property of another to oneself or a third person or appropriate means to:
(a) Exercise control over property of another, or to aid a third person to exercise control over property of another, permanently or for so extended a period or under such circumstances as to acquire the major portion of the economic value or benefit of such property; or
(b) Dispose of the property of another for the benefit of oneself or a third person.
(2) Deprive another of property or deprive means to:
(a) Withhold property of another or cause property of another to be withheld from that person permanently or for so extended a period or under such circumstances that the major portion of its economic value or benefit is lost to that person; or
(b) Dispose of the property in such manner or under such circumstances as to render it unlikely that an owner will recover such property.
(3) Obtain includes, but is not limited to, the bringing about of a transfer or purported transfer of property or of a legal interest therein, whether to the obtainer or another.
(4) Owner of property taken, obtained or withheld or owner means any person who has a right to possession thereof superior to that of the taker, obtainer or withholder.
(5) Property means any article, substance or thing of value, including, but not limited to, money, tangible and intangible personal property, real property, choses-in-action, evidence of debt or of contract. [1971 c.743 §121]
These laws would be applicable had intersango not voluntarily relinquished ownership of the coins. When you send bitcoins, you are no longer the owner of those bitcoins. They are therefore not your property and there is no legal right to them. Does everyone forget that these transactions are digitally signed? Intersango "agreed" by signing the coins away to bendavis that they are no longer the owners. Unlike mistakenly sending a tangible item that at the very simplest form you could simply "go and retrieve", once you are no longer the owner of the bitcoin, that's it and nothing can or will be done.
edit: in fact, since bendavis was the owner of the bitcoin after the transactions, i'd say intersango likely opens themselves up for a counter-suit for attempted extortion.