> Since ASCII characters and non-ASCII characters use the same concrete
> type (Scheme characters or integers) I thought it's simplest if we
> accept other characters.

The implementation is also simple and fast:

(define (ascii-control? x)
   (let ((cc (ensure-int x)))
     (or (<= 0 cc #x1f) (= cc #x7f))))

If we wanted to raise an error for non-ASCII characters, it would
require extra code. IMHO this settles the question: accepting all
characters permits both the interface and the implementation to be simpler.