We all want to side with the underdog. When there's a clash of overdog vs uberdog I suppose the overdog's position is still relatively under, so that's why I say "excelsior!" to Macmillan and "ha ha" to Amazon.
In the space of 48 hours Macmillan and Amazon went from minor border skirmishes to all-out war, declared by Amazon and lost by same a short while later. Ultimately it all came down to Amazon's vastly inflated idea of its rightful place in the affairs of man, finally clashing with a publisher big enough to say "no". Amazon tried to impose terms on Macmillan, Macmillan weren't having it and so Amazon withdrew its listings for every single Macmillan title.
Macmillan controls a lot of imprints. Suddenly, with no warning or reason given, a very large part of the global book supply was unavailable through the world's largest book supplier.
To save you crawling all over the interwebs for further details, Charles Stross has written a useful guide to the battlefield: Amazon, Macmillan: an outsider’s guide to the fight.
If you're more a bottom line sort of person, John Scalzi has written an entertaining analysis of Amazon's kamikaze strategy: All the Many Ways Amazon So Very Failed This Weekend.
As you may have gathered by now, Amazon lost.
I'm not published by Macmillan or any of its imprints: other than general principle, I have no declarable interest in this. But I am still smarting, nearly 10 years later, from the punitive discounts Amazon imposed on Big Engine in return for the privilege of receiving a basic competent service from them: one that listed my titles accurately, didn't unilaterally declare them out of print and so on. Now I feel those wounds healing. A little.