No, the capitalist way to fight a monopoly is to provide a better product and/or a better price, not to force your competitor to raise their prices to a point where you can compete with them through collusion.
So let's say there's 6 major publishers because I'm too lazy to look up the correct number. If apple had approached one of them individually and offered them the same deal and they had agreed everything would have been fine. But individually it looks pretty bad. The result would be that on Amazon (where 90% of the sales come from) the one publishers books would be available for $12.99 and 5 other publishers books would be available for $9.99 and they wouldn't compete. So individually the publishers would have turned them down. The illegal part comes in where Apple tells the publisher "Don't worry, we have talked to all the other publishers and they've agreed to the same deal so you won't have to worry about competing." Just like it would be illegal for all the CEOs to get together and agree on one price. It's not suddenly legal just because a third party is facilitating the price fixing.