Essentially, your neptunes are going to be pre-mined. You'll get it delivered at the highest difficulty ever seen. You won't be adding a 3TH machine to the global bitcoin hashrate, you'll just be re-attaching it when it's probably already been there for a couple of months.
Exactly. This is why cloud hashing is such a scam. They put the machines on the network and start collecting Bitcoin, then once the difficulty has adjusted a couple of times, they pay out the calculated amount at the higher difficulty and pocket the difference. Then, once a year or two goes by, they get to keep thousands of machines and the infrastructure paid for by the suckers.
The difference between CEX.io and KNC will be price. They will likely price the cloud hashing very competitively which will make it hard to resist for most customers. After all, they can afford to since they have already cleaned up by mining prior to selling and will keep adding hashing power to dilute your earnings.