Yes, you've got to be careful with using academic vs less than academic work. I too was confused at first, wondering how John MacArthur (president of a Seminary) would be counted as "popular" work but not academic work for research papers.
In one of my classes (at Luther Rice) after having written a paper, I realized that I failed to have good citations for the intro, but I had it for the rest of the paper. Oddly, the intro writing came last, not first. I didn't really want to slug through the academic literature yet again for the intro. I *tried* to flubb it and I used Thomas Constable's commentary figuring he was a Dallas Theological professor - surely, he would be okay to cite.
NOPE! POINTS TAKEN OFF! lol
Lesson learned!