Today I was listening to a recorded lecture about the history of opera, which includes excerpts from performances. The professor mentioned after playing one excerpt that he regretted he couldn't tell us who the singer was, because the record labels who provide the excerpts won't allow the course publisher to identify the performers. (He does identify the composer and the work for each excerpt, in order to integrate them into his lecture.)
The same professor has mentioned this practice online here and here, but not given a reason for it.
Of course the labels are free to ask for any provisions they want, and there's no strong reason for the course publisher to reject a deal with this condition, but why would the labels ask for this in the first place?