Apple was one of the first to legally sell digital music on the Internet. Apple wanted to capitalize on its early start, and wanted to reassure music companies that buying on iTunes would be legal and make money, so Apple created its own Digital Rights Management for songs sold through iTunes, called FairPlay. Songs from the iTunes Store can only be played on iTunes or iPods, and cannot be transferred freely to other MP3 players.
A proprietary protocol helps Apple make money because people must use iTunes or purchase an iPod to use the music they purchase from iTunes. It also assures artists and music companies that they will get royalties for songs to which people listen, instead of having people share songs for free.
Users complain of problems using the iTunes protocol. Most of the concerns are frustrations over the difficulty in using music they have paid for in the way they would like to use it.