There is no loss in quality and there is no upper limit on how many copies I can make from one master. I could post that on a web page somewhere, e-mail it to my friends, or do any number of things to propogate this gem in the wild. One CD, a few minutes, some bandwidth. Cost to you, a few minutes downloading to see if you like the song, a couple of megabytes of disk space. Some bands see this as free advertising, encouraging their fans to distribute bootlegs and ripped MP3s.
Some bands make samples or entire songs available.
Then again, some people aren't even that thoughtful. What's tricky is that many bands don't really own the right to music they've created. What happens is that their labels loan them money to make an album and sometimes even promise to pay them royalties after the expenses have been met. It's pretty expensive to keep your flavor of the month interesting. Meanwhile, people who actually spend more time writing songs and practicing their instruments than breaking up hotel rooms or trying to get acting jobs on TV or in the movies find themselves virtual wage slaves, having given over their right to control distribution for the chance that the massive distribution and promotion channel will shift some money their way.