You obviously know nothing about ebooks. The ebooks that are currently available only represents a small fraction of the total books available. And this small fraction represents some of the most popular authors. It would be naive to say that that authors who have published ebooks have no other offers.
Maybe you should read what I said again. I did NOT say "... authors who have published ebooks have no other offers...", however, the publisher usually owns all distribution rights unless specified otherwise in the contract, which prevents the author from submitting to any other publisher. The only exception to this might be if the author himself published the ebook version prior to selling to one of the print houses. Even then, the print house may purchase those rights, which would force the author to stop selling ebook copies.
Just because an individual might sell an ebook doesn't mean that it was commercially published by a publishing house. If it was, the the author is restricted by his contract as to how and to whom he might sell that same manuscript.