I think the rationale is that - at least as Prof Ng articulated in another ML course he taught this quarter - videos following a lecturer as he teaches on stage are actually _less effective_ than videos which just follow slides w/ audio narration. Another researched case was video lectures with the teacher's face in the corner vs. video lectures with only narration. In which they found that student's information retention was even better without _any_ on-screen presence at all. (Surprising, I know . . . I wish I had a citation, but I'm really just coming in on defense of the department's pedagogical choices here.)