define-syntax can only be used with local variables

[phiroc@free.fr]

Hello,
in the below code, nil! and nil2! only set variables values to '() and '(0) when such variables are local (let).
When I run the code with LispKit Go Scheme, global variables (such as y below) are also set.
What gives?
Many thanks.
Philippe



;;
;; ..\bin\kawa -C .\javafx0.scm
;; java -cp "..\lib\kawa.jar;." javafx0

(let ((x '(1 2 3))) 
    (define-syntax nil!
        (syntax-rules ()
            ((_ x)
                (set! x '()))))
        (nil! x) (newline) (display x) (newline))

(define-syntax nil2!
    (syntax-rules ()
        ((_ x)
            (set! x '(0)))))

(let ((x2 '(4 5 6)))
    (nil2! x2)
    (display x2) (newline))

(define y '(1))
(nil2! y)
(display y) (newline)

(define (f-nil! x)
    (set! x '()))

(define z 4)
(f-nil! z)
(display z)