Like practically every Greek tragedy you've ever heard of (and most of those you haven't), Prometheus Bound is obsessed with the difference between fate and free will. (These questions have been known to cause acute mental dizziness in generations of readers—so don't be alarmed if Prometheus Bound has the same effect.) Here's the issue: most of the characters express the view that everything is fated. And yet, most of them also keep telling one another to do various things. But why tell people to do things if they're fated to do them anyway? And why waste your time complaining or asking for help if there's nothing to be done?
The issues of fate and free will are more important to gods, because humans are too weak to have free will.
If fate rules everything, then Hephaestus's claim that he's just following Zeus's orders is accurate. On the other hand, if humans can act with free will, then Hephaestus's words might be an empty excuse.