cons は built-in 手続きだから。

R5RS 6 Standard procedures
> A program may use a top-level definition to bind any variable.  It may
> subsequently alter any such binding by an assignment (see 4.1.6).
> These operations do not modify the behavior of Scheme's built-in
> procedures.  Altering any top-level binding that has not been
> introduced by a definition has an unspecified effect on the behavior
> of the built-in procedures.