Anyone interested in seeing what the book is about can check the (slightly outdated) preview here:
https://minireference.com/static/excerpts/noBSguide_v5_previ... (a more descriptive title would be high-school math, mechanics, and single-variable calculus three-in-one)
To be honest, I think the book is used widely because of the title. There are many (hundreds, perhaps even thousands) of good math books out there with standard titles, and from what I have seen, most learners will not bother with them, even though they are totally capable of understanding math AND the books would do a decent job at explaining.
The irreverent title gives me just one extra moment of attention with the cold-reader, and if I can capture their interest in that moment (the first section also has a swearword in it), and get them to bring their "math guard" down for fifteen minutes, then I can "convert" them to the beautiful subject.
You're right that the same "feature" of wow-this-book-seems-to-be-different is also a turn-off for most people who are already pro-math. This cannot be serious math if there is a swearword in the title. Some readers have said they get the sleazy "learn math in three-easy-steps feeling!" when they see the title. That's just life—you win some you lose some ;) I am less worried about people who already like math, since they, having already overcome the fear of math, can learn all the other good books out there.
Yes, I don't see why not? Maybe this is a cultural thing, but I don't know anyone who would be offended by the word bullshit, professionally or otherwise.
Would recommend it to anyone whose maths needs a bit of a brush up, or anyone who's interested in basic mechanics!
Oh and I'm using Khan Academy for extra practice, which I can warmly recommend as well.