In Israel they are advertised specifically for that purpose. I'm not religious, but I suspect that this is exactly the case. However, since this user case is non-orthodox for heating surfaces, I think that they need special testing and quality control for hot plates sold for this.
My mom just told me about that story, and when I came to HN I thought it had to be related that this made front page on HN exactly when this tragedy happened.
TL;DR: Tragic fire caused by a hot plate left on overnight so that it could be used to warm food on the Sabbath.