When the PM doesn't stop the customer from requesting a bad feature (in terms of user experience / design)

Thats where its good to have BAs to work directly with the Product Owners to refine the requirements and make sure they are getting what they really want. User stories should be the result of collaboration between us and the business, not the business saying "we want this plz". And even then stories are not prescriptive about how it should be implemented. They are the promise of a conversation, not a set of instructions to be performed by a dev monkey.