To be fair, if I published something from Socialist Worker (does it still exist?) I'd reference it. I usually even reference any mainstream quotes since every news source has its biases.Why? So you can attack the person and not the opinion.
Breitbart is, in European terms, a far right libertarian news source. (Just as The Sun is what it is, the Guardian is what it is, and New Statesman is what it is.) It doesn't mean that what they publish is automatically right or wrong but it puts the publishing of the article in context.