This isn't about deciding whats in their store, its the justification given. In the US, an at-will employee can be fired for any reason, excluding special cases

This isn't about deciding whats in their store, its the justification given. In the US, an at-will employee can be fired for any reason, excluding special cases like sexism, racism, or homophobia in some states. A store can decide to not stock a product for any reason, except for a set of special cases. I think that 'because you are also friendly with a competitor' is one of those special cases. I would definitely have a problem if amazon refused to stock books that are also on B&N, or if Netflix refused to provide a movie only because its also on HBO right now (note that this obviously doesn't apply if there were exclusivity agreements made with the consent of the content maker).