Let us remember what has happened to get Poseidon so angry. Poseidon has been the god who has been opposed to Odysseus, just as Athena has been supporting him. In Book 13, we are presented with a Poseidon who is incredibly angry about Odysseus's rescue yet again. Thus he goes to his brother, Zeus, king of the gods, to ask for permission to punish the Phaeaceans for the help that they gave Odysseus. Zeus agrees, though not allowing Poseidon to go as far as he would like in his punishment.
Essentially, therefore, the gods are shown as vengeful, fickle and capricious in this book. Note how the Phaeaceans are punished even though it is destined for Odysseus to arrive home safely. Thus although they are giving a helping hand to destiny, they are still punished by the gods because of spite and the fact that Poseidon is working against Odysseus. Yet again, humans are shown as the playthings of the gods who suffer randomly according to the whims of their masters.