User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5247339-20130520192736/@comment-3146930-20130521004220

It is mentioned that because gods are immortal, they get off lightly when it comes to broken promises on the River Styx. When a mortal does it however, they are punished in this life or the next. However, no broken promises on the River Styx have been seen so far in the series. Also, pretty much any demigod is going to have a hard life; especially children of powerful gods. Even if they weren't the ones that broke the oath and it was somehow the gods' fault, someone always has to pay for a broken oath.