It is worth adding that though I agree that songs can be literature, songs also have non-literary elements, which means that songs that are not literature can still be good songs. Here I'm thinking of artists like DJ Shadow or the Beastie Boys, where some other element such as the soundscape or even just the beat carries the music.

I'm not a Bieber fan, but I think it's unfair to judge him strictly by his lyrics. Or Sinatra for that matter.