The difference is who is selling what, and who is making the profit.  When an artist goes on Leno to promo an album, they are selling themselfs and their music, and the processs and profit is more or less in their control.  The objections come - especially on older music, or an artist's early releases - where the record company owns the performance, and the publishing company owns the copywrite.  In those cases the artist get's nothing, and has no control over the song's use.