My speculation:

You could think of it in comparison to attitudes toward "natural resources": are they something merely existing as resources to exploit, or do you respect them for what they are?

You could see the use of the term either way: these people provided source material (and the source material is the stuff of interest), or that sense of "source" could be meant with more respect, as in, we'd be nothing without these people who provided us with the songs.

I can understand some discomfort with the possible ambiguity. Perhaps Norma would prefer consideration of these people as singers in their own right, rather than sources of songs.

~ Becky in Tucson