Unfortunately, there was also Rule 12. Rule 12 informed the students that they were not allowed to care about Thing X. (Yes, that's a paraphrase. No, I'm not going to link to the actual list.) This was pretty clearly code for, "Don't write speculative fiction stories." In other words, if you want to be a good writer, you can't be writing spec fic.
Look, I understand that nearly everyone has that one section of the bookstore that just doesn't get read. I'll be honest - I don't think I've ever bought a western, and I don't see that changing in the near future. Having said that, I have no problem understanding that by not reading westerns I am missing good fiction. It's just that there are a lot of books that I want to read, and my time in which to read them is finite. There's going to be some stuff, and some really good stuff, that I'm going to miss. But there is a difference between understanding that a particular genre isn't your thing, and dismissing the entirety of it as not worth caring about, and hence not worth writing in.
A casual reader gets to make those distinctions. A teacher does not. It's one thing in, for example, a course on playwriting, to insist that your students turn in something that is a play, and not a sestina. But a good teacher understands the distinction between something that is not the sort of story that he would read for entertainment Saturday morning, and something that is genuinely poorly written. And a teacher who believes that all of Genre X is poorly written needs a new required reading list.