The book's toxic mix of purism, atavism, and personal eccentricity is not underpinned by a proper grounding in English grammar. It is often so misguided that the authors appear not to notice their own egregious flouting of its own rules. They can't help it, because they don't know how to identify what they condemn.It's a wonderful take-down, the best I've encountered since Louis Menand's examination of Eats, Shoots, and Leaves ("Why would a person who is not just vague about the rules but disinclined to follow them bother to produce a guide to punctuation?").
The power and popularity of such books is astounding. Both books were quite popular in the high school English departments I worked at, and Strunk & White was often assigned to students. I'm not against teaching basic grammar (I've done it most of my years of teaching), but if you're going to teach grammar, don't teach from books that are, as often as not, incontrovertibly wrong.