There's an interview of Abelson floating around where he is asked why there was so much calculus in the examples and exercises. His response was that it was written for students who had just finished multiple semesters of calculus, so it made sense to use it. He goes on to indicate they made a second version, handed out as class notes to students in art degrees where the calculus was all replaced.

I also think you are introducing functional programming the wrong way to beginners. It's best and most intuitive use imho is in a math class. For example this book: https://cs.wheaton.edu/~tvandrun/dmfp/ has little prereqs, and will teach the reader both programming and math regardless of educational background. It really, really shines when it comes to learning counter-intuitive things like cardinality. I'm not the only person who thinks so either, here is one of the curriculum designers of Bootstrap, a US highschool programming curriculum, explaining how naturally that style of programming fits with algebra classes. https://www.janestreet.com/tech-talks/curriculumn-design/