Apple owns a mall (iOS). In that mall, Apple owns a book store (iTunes). Amazon also has a book store in the mall (Kindle).
Apple does not want to sell Amazon's books (Kindle format ebooks) in it's store (iTunes). Ok.
Apple decrees that no store in the mall shall sell any books but iTunes books, which only they may sell. Not Ok.
In the brick-and-mortar world, malls are fungible, and stores that sell the same goods are fungible. The iPhone is not fungible.
Anyone that wants to constrain themselves to a single market, needs to accept the rules of focusing on a single market.
Apple owns a mall (iOS). In that mall, Apple owns a book store (iTunes). Amazon also has a book store in the mall (Kindle).
Apple does not want to sell Amazon's books (Kindle format ebooks) in it's store (iTunes). Ok.
Apple decrees that no store in the mall shall sell any books but iTunes books, which only they may sell. Not Ok.