There is intentional and purposeful use of other people's material, like in the case of many hip hop and rap songs. They will first ask permission to use another song –- like the bass line from David Bowie's "Under Pressure" for Vanilla Ice's "Ice Ice Baby" or Eminem using Dido's chorus hook in his song, "Stan." The original writers will get a writers share of the new song.
I don't think it was Nevin's intent to write his LA song to the melody from Elton John's song "A Candle In The Wind," but it's certainly a good lesson to make sure you guard against such things in the future. It can be a costly mistake.
The other thing I saw contestants grapple with on this episode was the fact that we judges are looking for REAL hits for OTHER artists -- not hits for themselves as artists, and we don't care how they sing. They need to sell their songs and that's it. Countless great songwriters can't even sing, while others like my co-judge Kara, can sign their tails off. BUT we are listening to the bones of the song and if it's viable for an artist, not if they are great vocalists.
We judges came down on them hard because the kind of song you would write for yourself as an artist, or that you could imagine for another singer-songwriter, are very different than the actual hits non-writing artists are looking for. John Mayer can write a quirky song, like "Your Body Is A Wonderland," and have a hit with it. But singer-songwriters write their own material. They don't need outside writers to write what they are already doing. However an artist like Usher needs a straight up hit that will have kids dancing in the club, and it's a Carrie Underwood type of artist that needs a fresh, well-crafted lyric on a song built to showcase her vocals.
Hit songs need to be clear and recognizable, unique and universal at the same time, like a great sculpture, so they stand out on radio, and so artists want to cut them. We are not looking for middle of the road decent songs that might make a record but never the radio. A writer would starve this way if it was all they wrote for other artists.