The reasoning is that sheets have a high thread count, which forces your quilting needle to break the sheet's threads as it pieces through. This supposedly leaves holes in your backing and diminishes the stability of your sheet. Your quilting fabric has a looser weave which allows your needle to easily slide between the threads and keeps your backing fabric intact. This doesn't mean that you don't have a choice it just means your quilt will last longer according to the piece I read. Just be a happy quilter.
Mia