WTO rules say that trade restrictions cannot be placed on products because of the way they have been produced. This severely constrains enforcement of production standards meant to protect the environment. WTO rules explicitly recognize the importance of environmental concerns. The WTO Appeal Body accepted the right of the US to ban use of shrimp caught without TEDs and only said the existing law was too broad. Environmental agreements rarely include any enforcement mechanism.
Trade restrictions are often the only effective sanction.
Any restriction concerning TEDs narrow enough to satisfy the WTO would be too weak to protect sea turtles effectively. The WTO is a trade agency not an environmental agency. These are among the very poorest nations of the world and WTO rules allow them special consideration. For instance, the Uruguay Round agreement allows them to reduce their trade barriers by as much as other members must. Rich countries have higher barriers to imports from developing countries than to imports from other rich countries.