Having been a TA for CS1 classes, I can understand the rationale behind it to some degree. It's ridiculous how many students go online and ask for someone to do their homework or even copy chunks of code off of stack overflow. Forcing them to explain every line is one way of judging how well they actually understand what they're writing (if they do at all). Of course it's going to be annoying to anyone who already has the slightest experience... but what can you do?

> It's ridiculous how many students go online and... copy chunks of code off of stack overflow.

You could easily replace "students" with "coders" and that statement is still true. Just an observation.

For the first programming courses I can understand the commenting, but I've been TA for advanced courses where many of the students have passed the compiler class. They don't need comments anymore, and it really detract from the readability of the code.