In a way though, isn't that like anything? I would say things like a high quality instrument, an instrument itself (built and invented by others to make good sounds), a high quality microphone, and a knowledge of theory are similar to "source material." There are also other factors that don't even relate to the sound of an artist's music that could be seen as leading to injustice in the "reward" the artist receives- chance, privilege, personality, etc. Finally, I don't think an artist will get very much critical acclaim if their sound owes too much to its source material, because people can notice that stuff, and the more people who listen the more likely someone will notice.
So, overall, I think it's probably good that artists are paid royalties where they are due. But the concept of intellectual property shouldn't restrict any type of expression. "Where they are due" is where emphasis should be placed, and I think generally, not much is due, especially if not much money is being made off it.