The point of the example is to explain pointers by starting with a more familiar concept: indexed arrays. It would help if you were referring to the book; the authors note more than once that "the programs are intended to be illustrative, not bullet-proof."

Understanding the precedence and order of evaluation of operators is fundamental to mastery of any programming language (chapter 2.12 in K&R). All C code must "rely on" the order of operations, and if an indirect assignment through a pointer with post-increment is confusing... well that's the point I was making in the first place.