I'm not positive about this answer, but it is how I do it. If I am the purchaser of the music, I can put it on any of my own devices and CDs for my own use. I can't give it to other people or sell it. That seems a little wierd because used CDs are sold at garage sales and in music stores. Used books are big business. Why is digitally downloaded music different. I think we are in a transition period and making things up as we go along as best we can on this issue.
Is it good that musicians and authors are finding it more difficult to make the big bucks that they did in years past? Is the digital age leveling things a bit?
Most musicians make thin money on tours and merchandise and much less on the sales of the music itself. Also, with the sale of a CD there is still only one owner. Digital songs can be multiplied without any more compensation to the artist or label.