From the Lightning Network paper:
... once an updated commitment transaction is agreed upon, the previous commitment transaction pair is revoked by sharing the private keys needed to redeem those encumbered outputs. Thus, A shares its (throwaway) private key, and B shares its throwaway private key. If A were to sign and broadcast a revoked commitment transaction, B could not only immediately spend its own output, but it has both A's key and its own to generate a transaction which can spend the output which normally go to A after a delay.
It's not clear to me how the throwaway private keys should be shared between the two parties. What if Alice shared her key to Bob but Bob refuse to disclose his? In that case Alice cannot spend the earlier commitment since otherwise Bob can steal her time-locked funds. But Bob could still spend his previous commitment without worrying his fund be revoked by Alice.
Is it considered an unfair situation? And how the protocol address the problem?