It's reported that in March, Amazon asked music labels to give it one day of exclusive rights to sell some songs before they were widely available. In exchange, Amazon offered to include those songs in its "MP3 Daily Deal" promotion.
Apparently, Apple wasn't thrilled about the arrangement and requested that the labels refrain from participating. The company also allegedly punished those who did partake in the deal by withdrawing marketing support on iTunes for the songs involved.
Apple is the largest music retailer in the US, with over a quarter of the total market and two-thirds of the digital market. Apple, Amazon, nor the DOJ has responded to the news.