Well the alternative to "here's a service that converts a zipcode to a polygon, and coordinates to a list of polygons, and here's the fine print of limitations and how they are communicated" is declaring that postcodes are arcane entities and their connection with the mortal plane is only an illusion, and no such service is possible.

If I was developing something that required dealing with US GIS I'd very much prefer a service that does the above, and handle the edge cases, than have no such service at all.