These cretins have given themselves license to belch this stuff due to a convenient belief in the journalism community that since there is no such thing as "objectivity" one is free to propound whatever flat-earth nonsense one "feels" is appropriate any time one wants. And what is worse, this crap sells - and why not, they can force you to buy anything they want if it's a textbook. And still worse - they are comfortable doing this because so many others do.
If you feel like making trouble - I wouldn't recommend it, they get to decide if you get your piece of paper - but if you do, remind them that "diverse sources" MUST include conservative ones and challenge them to prove that they've included any. It's evident they don't bother to live up to their own benchmarks here, isn't it?