"Threatens" is the operative word. The threat is often enough to dissuade someone from exercising their rights. In reality, she is not going to pay a lawyer to try to defend her copyright where it doesn't apply. One thing is that the lawyer is not going to take it unless she pays up front. They know she can't win. She know she can't win. So, she threatens. (I know nothing about the designer you are referring to - I'm speaking in generalities).
Someone who vigorously guards her copyrights is Eleanor Burns. She carefully guards her patterns. If you want to make a quilt from one of her patterns, you just about have to buy it. If someone posts one online, she has it removed. She has a perfect right to do that. Her pattern is her creation. As far as I know, she makes no effort to control the people who use her pattern to create quilts.
I'm not sure why anyone would want to control the sale of quilts made from their patterns, but their copyright doesn't extend that far. It only covers the sharing or selling of their product - the pattern itself.
bkay