Sony BMG said this afternoon that it would start selling music in the MP3 format through Amazon.com. That makes it the last of the four big record labels that have abandoned digital rights management software, which limits how many times copies can be made of digital music.

Amazon, at this moment, is the only place that can sell nearly any song without copy protections. (But it would be no surprise if Apple announces the same when Steve Jobs gives his keynote at the Macworld Expo Tuesday.)

For Sony, this move comes after a bit of a head fake. Word of its shift away from D.R.M. came first through a Business Week article. But earlier this week the company came out with a confusing half measure: It announced that it would sell cards in retail stores that would allow users to download digital versions of albums in a form without copying restrictions.

Some took this to mean that this would be the only way that Sony would support unrestricted content. Sony officials have as recently as this week told music executives in private that this interesting but modest idea was the only non-D.R.M. music sales plan it had for now.

Now Sony is doing what is really the only logical course at this point: it would be confusing to everyone if three out of four big labels (and most independents) sold unprotected music, but songs from Sony still had copy restrictions.

Then again, logic has been elusive in the music biz.