The print and radio pool also distribute their information to other outlets. Stills use their respective wire services to distribute images. WH has invited secondary and new media outlets into pool that do not have any responsibility to provide content beyond their own viewers/readers. Those outlets do not provide a collective product.
The 5 networks expend considerable resources for staff, equipment, travel costs to provide coverage of the president anywhere in the world and have an established infrastructure for how the images, audio and editorial information is distributed for use by other outlets. That is how the public receives this coverage.