When Gnomecopter Tours was released, this song became available to free-to-players as well. However, when Jagex decided to remove the Gnomecopters, three songs (Background, Legion, and Narnode's Theme) were removed to make a cap of 150 songs for free players.
There used to be a glitch in which if the player walked a bit south of the Lumbridge mill, the song would play and unlock, making the song available to free players. This was also the same with standing on the edge of Crandor and unlocking Dragontooth Island. This has been fixed.