It's illegal to steal anything, virtual or otherwise.
So you're claiming that there is such a thing as software theft, as opposed to copyright infringement then?
You don't need a new law for anything tangible. But bitcoin isn't a tangible thing. It's a protocol. It needs a law or precedent before we can say whether there is such as thing as stealing "bitcoins".
OTOH, we've already seen lawsuits.
Lawsuits can be issued for anything, regardless of the law. A written law, or convictions under an existing one are the only things that could show that there's a crime of "stealing bitcoins".