It's not illegal. It's not any more illegal than me borrowing your chain saw to cut wood and selling the wood out from the back of my truck.
What the US needs to do is make it so that game companies can claim their games and the virtual things in them as assets. Then taking assets from the game to sell to others without consent could be considered theft. But that's getting the government to acknowledge virtual (non existent) things as being tangible (having real world value) which I don't think will happen anytime soon.
