I think there's a third explanation, which is that computer science professors are made to teach computer programming courses to students who want to learn computer programming and not computer science. If you're a skilled computer programmer when you enter the course, you're probably already more knowledgeable about computer programming than the computer science people who are supposed to be teaching you.

I'm sure my professors knew their computer science niches just fine, but they knew very little about programming & software engineering. Programming was my main hobby growing up. In high school I was already using Linux, had written dozens of websites for myself and friends, had written a 3D platforming game from scratch, and had made and published several homebrew games for real video game hardware. Having to then spend a semester "learning" C++ and operating system basics, from someone who has spent their whole life in academia and never published any real software, was godawful. They barely knew what they were talking about and made all kinds of beginner mistakes. I definitely knew way more than my professors about the subject I was there to learn, computer programming.