Sure, the owners of the mining hardware could be on the other end, but that puts up the cost of the attack substantially - instead of depending on the cost of renting an hour's mining time, it now depends on the total lifetime value of the equipment used. There's also the rather large problem of how they could convert enough double-spent Bitcoins to something else before the price tanks; it'd have to be a very big heist to make the math work out.
Well they don't have to be on the receiving end of the attack. They can be, you know, on the other end.