Short Sales

My sister is purchasing a house in Colorado, but I think this question may have implications all over the country. She writes: In a short-sale, why does the owner have ANY say in how much offer they take to the bank? I mean they are not going to get a dime of it anyways (their mortgage is way more than the offer), so why not take the offer, let it go to the banks, which extends their time to not foreclose and gives them more time to get more people in to see it and make other/better offers..... it would seem to me that they are in a win/win just to take the offer and let it go to the banks. Why would they negotiate a higher amount???????