Effective today, Warner Music Group (NYSE: WMG) follows Universal Music Group and EMI to the DRM-free Amazon (NSDQ: AMZN) MP3 store. For Warner, it’s an admission that DRM-free music is part of its present and future; for Amazon, it’s a major influx of titles for the store launched in September and a boost to its competition with Apple (NSDQ: AAPL) iTunes. While titles from Warner are supposed to be in the store today, it’s unclear how many songs and albums are involved. The release simply says “songs from WMG’s digital audio catalog.” In addition to the a la carte sales, WMG will team with Amazon to provide album bundles with exclusive tracks and other “digital music products.”

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Why Amazon? The long answer from WMG exec Michael Nash in the release really boils down to “it’s not Apple”—although not in those explicit words. There’s also no mention of financial terms. Amazon sells more than one million of its nearly 3 million for $0.89 per song with the top 100 albums running for $8.99. Amazon’s holiday sales press release tossed around lots of numbers but nothing specific about MP3 sales./ Release.