Encontrei a resposta na entrevista da MortalNet com Rick Riordan :
TK: There are many questions surrounding the River Styx. For mortals and half-bloods how much power does a promise on the River Styx hold over you other than the death penalty?
RR (Rick Riordan): Well the Fates oversee that, and you never want to mess around with the Fates. When or where they will they get their revenge should you break the oath that’s anybodies guess. Suffice to say you don’t want to find out.
SK: Does that go for the gods too?
RR: The gods do swear on the River Styx and they do get in trouble, but they tend to get off easier because they are immortal.
TK: How were Zeus and Poseidon able to break their oath not to have any more children?
RR: Well unfortunately the gods do that all the time and really the doom, the curse happens with their children. You can break your word it is just not a good thing to do.
TK: Didn’t they swear on the River Styx, which is unbreakable? Does this mean that the gods can break Percy’s promise and not claim their children?
RR: Well that’s the question that will have to wait for the next series to answer. That is an excellent question.
E os deuses quebraram sua promessa com Percy no primeiro livro dos heróis do Olimpo (Jason não foi reivindicado até os 16 anos)