This is not a reason to prefer Macmillan over Amazon. Amazon might arguably have something that is almost a monopoly on online bookselling, allowing them to de facto dictate the price of books purchased online (though not offline, except to the extent that price wars dictate that competitors charge less).

Macmillan has an actual monopoly, guaranteed by Congress, on producing Macmillan books. Macmillan is telling Amazon to engage in actual price fixing. Macmillan has told Amazon that if Amazon does not set their prices to exactly what Macmillan says they should be, when Macmillan says this should be, Macmillan will use its actual monopoly to forbid Amazon from distributing their books.

Macmillan is doing this to avoid channel conflict -- i.e. establish actual price fixing backed up by the power of law to forbid there to be price-based competition among the various outlets who sell their books.

It turns reality on its head to portray Macmillan as the pro-consumer champion of a vibrant, competitive market here.