Even if it was a difficult operation to copy a song, it only takes one person to do it. After that the spread of the song via the Internet or other means of propagation is only limited by the honesty of the users.
“I went to work in 1962, and by '64 I was writing all the time, every night and every weekend. It didn't occur to me that, having read nothing and knowing nothing, I was in no position to write a book.”