(a) Many do. In fact, many old paperbacks had order forms in the back for other books directly from the publisher. Been doing it forever. On the other hand, you will pay full cover price, which for ebooks means full hard-/paper-back price. Because...

(b) The publisher's other retail markets (Barnes & Noble, Wal-Mart, other distributors and bookstores) would crap out a literal brick, which they would then apply to the publisher, if the publisher started competing with them.