A number of books also make the same mistake.
Nobody knows for sure.
The Industrial Workers of the World seeks to organize workers along industrial lines as opposed to craft lines. The idea is to organize all workers in the same industry into a single union. Furthermore, historians and reporters often associate the IWW with Communism, even though the IWW is not a Communist organization. The IWW is a revolutionary industrial unionist organization, and its membership includes radicals of every stripe, including anarchist, communist, socialist, syndicalist, etc.