The designation "Western" on the spine of a book really means very little.

In the early days of the paperback book industry books were categorized by publisher, this helped the distributors to quickly restock the racks and certain publishers did specialize in different types of books, sometimes different genres.

Then in the early 1960s, in an effort amusingly called "integration", booksellers and publishers combined the placement of all their titles into genres or categories. At that time the pressure for a writer to stick to one genre, or for all of that writer's work to be categorized into the genre they were best known for became enormous. With thousands of paperbacks for sale in even a modestly sized bookstore how else were you going to find the work by your favorite author?

No matter the genre writers work was generally lumped together in the genre they were known for and publishers just stuck the genre designation on the spine of the book to be sure the distributors got them all together.