So the cosmos is at risk of repeating an endless loop of gods and goddesses undoing each others’ work due to their own petty obsessions, but there’s a bigger problem. The gods are specialized in nature. Artemis is goddess of the hunt, Apollo the sun, Demeter of agriculture, Poseidon the sea, etc. These specialized domains belong only to those certain gods. Yes, there is some overlap. Hera and Zeus both have prophecy. Apollo shares the hunt with his sister, and he and Helios share the sun.
However, there are subtle differences in the expression of these domains and these domains influence every aspect of the god’s powers. While any god may transform a human, the nature of them is always according to the nature of the god’s powers. Demeter transforms Erysichthon by giving him endless hunger, a power related to her as goddess of agriculture. Artemis transforms Acteon into a stag, related to her as the goddess of the hunt.
Each god and goddess must remain the absolute ruler of his or her domain in order for the whole system to work. Part of what Zeus has done is establish order over chaos. All of his myriad affairs with other goddesses serve to bring order out of that chaos. All of the offspring from these unions further refine the powers of the various gods in the pantheons and enrich the world with more specificity. Instead of a handful of deities broadly embodying every nature of the world, he’s created the means for there to be a completely ordered hierarchy to how the cosmos functions.
In reality, no god is fully capable of undoing what another does. The individual natures of the gods affects the exact function of their powers. This is why Zeus could not make the crops grow when Demeter went on strike. He didn’t even try, either. He and Helios could have paired up to supply the right conditions of sun and rain to facilitate grown of crops, but something essential is missing. It’s like trying to speak a language fluently using a phrasebook. Agriculture is an extension of Demeter’s being. Storms are an extension of Zeus. Apollo and Artemis embody different, albeit similar, aspects of the hunt. Athena and Ares share war, but they do not completely overlap. Ares’s brutality differs from Athena’s strategies.
So not only is this a law established by Zeus, it’s to prevent an unmitigated disaster of gods even attempting to undo. Because it wouldn’t work, and the result would be even more chaotic.
So, Hera blinds Teiresias, related to her power as goddess of prophecy. And Zeus, while not directly able to undo what she has done, further transforms T by giving him the gift of prophecy. Hera, Zeus, and Apollo all share prophecy as part of their domains, just different aspects of it. Zeus is able to gift his particular brand of prophecy to T. He can no longer see, but he has the future sight of oracles to help compensate for what was lost.