Honestly, because the very first sentence of the preface is "This book aims to provide an introduction to the topic of deep learning algorithms." Really? LOL. If you're going to pitch 600 pages of dense mathematical notation as "introductory," you're going to have to expect some people to call BS.

What's interesting/unfortunate is that their Python code samples really are easy to follow and pedagogically useful to a beginner. I think a lot of people will be turned off by the text unnecessarily.

It should have been promoted as a rigorous reference textbook, which is what it is, and not any sort of tutorial or primer.