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]
Each line describes clearly that the "thief" must have taken an item owned by another individual, or withheld an item owned by another individual.
Here is the question and answer from the "How does Bitcoin work?" section of the Bitcoin FAQ, published on the Wiki linked from Bitcoin.org:
Q. How does Bitcoin work?
A. Bitcoin utilises public-key cryptography. A coin contains the owner's public key. When a coin is transferred from user A to user B, A adds B’s public key to the coin, and the coin is signed using A's private key. B now owns the coin and can transfer it further. A is prevented from transferring the already spent coin to other users because a public list of all previous transactions is collectively maintained by the network. Before each transaction the coin’s validity will be checked.
The relevant line here is indicated in red.
Ben, having been SENT the coins, whether mistakenly or not, now owns them. The network was informed of the transfer of ownership by phantomcircuit and then confirmed the transaction by including it's data in the blockchain.
Yes, what he did was morally wrong. Yes, what he did was ethically wrong. No, what he did was not illegal.
He's a dick. Plain and simple. But he is well within the bounds of legality in both Oregon State, and the United States of America, where he resides.