Correct, an amount cannot be viewed on chain without the view keys. There was a talk at Monero Topia yesterday where the guy said that the way it works is that Monero uses range proofs to prove that a value is a positive number and less than 2 to the power of 64. This has to be done because if there were a negative value the equation would still balance but a small negative number would generate a positive number of like 2 to the power of 200 and let you inflate the supply.