The exchange cannot be manipulated by false orders.
I'm not sure I entirely follow. There are people who have discussed emptying their bank account so as to cause the Coinbase ACH to get a return code for NSF, after buying at a higher level and then later seeing the BTC/USD exchange rate at a fraction of that level.
That essentially is a fake order, and Coinbase then is left holding the bag on the expensive bitcoins the buyer bailed on. Additionally, Coinbase incurs a fee I believe for that NSF.
So they definitely take a risk with their current model.