they make money by mining bitcoin, if the value drops then their return, even if they sell all BTC immediately, would be affected. They just spent $100m on a new mining facility so to trash bitcoin would be to write off that investment. They cannot make good money by screwing over bitcoin without screwing themselves over.

Best case scenario is that at the end of the effective life of this DC BitFury decide to doublespend their coins to allow them to spend all bitcoins once, however by that time their relative % of the network will be less than 50%.