Good post Fremont.
I think the owner of a work is always the person who wrote it, and in the first instance they own the copyright (= the exclusive right to make copies), but they can assign that copyright. As Gary says many composers assign the rights to a publisher (i don't think it's legal to actually sell the copyright any more).
A publisher gets the copyright, often in return for paying an advance on future royalties, and gets a share of those royalties (in the UK this can never be more than 50%).
Under some contracts a publisher takes a relatively low percentage (15% - 20%), in which case they are really just taking a commission for collecting royalties from various agencies round the world. When they take a larger percentage they generally undertake to actively exploit the work, e.g get bands to cover the song or get it placed in movies, TV or commercials.
The main bulk of my living is from music that is published, the publisher gets 50% and I'm very happy as they exploit the works very well, 50% of a lot is better than 100% of not much.
But my contract is not exclusive, so for other music I write I can retain the copyright. This is music I make available for download, and I either charge for it or distribute it for nothing. That is my choice.
Usually artists who allow free downloads are doing it as a promotional tool, it gets more of their music "out there" so gets a wider following and more people likely to turn up to their concerts, which then becomes the main source of revenue.
One argument often used by people who think they should not pay for downloads is that it helps the artist anyway. Whether this is true, it should still be the artist's right (IMO) to decide whether to sell or give away the music.
For reference, the site I use for downloads allows me ("The Artist") to choose the following types of creative commons licence as an alternative to "All Rights Reserved"
Attribution Non-commercial No Derivatives
This license is the most restrictive Creative Commons license, allowing redistribution. This license is often called the "free advertising" license because it allows others to download your works and share them with others as long as they mention you and link back to you, but they can't change them in any way or use them commercially.
Attribution Non-commercial Share Alike
This license lets others remix, tweak, and build upon your work non-commercially, as long as they credit you and license their new creations under the identical terms. Others can download and redistribute your work, but they can also translate, make remixes, and produce new stories based on your work. All new work based on yours will carry the same license, so any derivatives will also be non-commercial in nature.
Attribution Non-commercial
This license lets others remix, tweak, and build upon your work non-commercially, and although their new works must also acknowledge you and be non-commercial, they don't have to license their derivative works on the same terms.
Attribution No Derivatives
This license allows for redistribution, commercial and non-commercial, as long as it is passed along unchanged and in whole, with credit to you
Attribution Share Alike
This license lets others remix, tweak, and build upon your work even for commercial reasons, as long as they credit you and license their new creations under the identical terms. This license is often compared to open source software licenses. All new works based on yours will carry the same license, so any derivatives will also allow commercial use
Attribution
This license lets others distribute, remix, tweak, and build upon your work, even commercially, as long as they credit you for the original creation. This is the most accommodating of licenses offered, in terms of what others can do with your works licensed under Attribution.