Plus, it was a shared mythos. He created the main core of it, but others contributed as well. So, figuring out copyrights would be difficult, since some stories reference other stories. Remember, they thought they were just writing some horror stories, but that they would never be something anyone other than other horror story enthusiasts would see.
IP was not a thing yet, and they were just writing for fun. A modern version would be the shared SCP universe.
Um...you can literally see the Starbucks logo on the side of the building.