<MODE: Devil's Advocate>
The issue is entirely legal - The state decides that affording a monopoly power over songs (or books or arts) allows for the artist to get paid and gives an incentive for her to produce more songs. Notice that there is no similar rule over paintings/statues, anyone can make a copy of the original. As long as the original is not stolen the state does not interfere.
I heard a song, now the song is in my mind. What moral right does someone have over me singing that song ? If someone does not want me to sing a song, they should not sing it while I am around. Keep it a secret or sing only to their friends or something.
I would even say that if the copied song sells more than the original (and other copies) then the copied song is an improvement over the original and hence a valid contribution to the arts. For eg. I can go around singing "Aafreen Aafreen" and other than a slap from girls at the bus stop, I won't gain anything from it. On the other hand Nusrat Fateh Ali Khan can sing it and make money from it - What moral right do I have to stop him from earning his bread (if I were the composer of the song)?
How is free speech different from free music ?