Erm, part of the key quantum key setup process requires a classical channel after transmission in order to exchange information about the quantum bits which were just sent. This isn't just about some password being whispered in advance. If you're talking about some other algorithm, e.g. for general secured data transfer, could you give more specifics?
The classical exchange serves to authenticate some of the qubits that were sent, and those qubits are emphatically NOT used to generate the key. The qubits used for that purpose are not exchanged through the classical channel.
Regardless, classical crypto is about the strength of encryption, and cares little for people reading ciphertext. The quantum crypto promise is of a totally different flavour, promising physical obscurity. If its response is "well of course we can only guarantee that Eve is not intercepting once we have guaranteed that Eve is not intercepting!" then, etc.
More specifically, it guarantees that if Eve intercepts the message, you will know it, and therefore you will throw away whatever OTP you have generated without using it. Yes, this means Eve is able to completely break communication (which she could also do, for example, with an axe). What she cannot do is intercept while staying undetected.
