Why does formattedRead take a non-const ref?

[Andrew Godfrey via Digitalmars-d]

The first parameter of formattedRead is a non-const ref. Is there
a good reason for this?

e.g. the below doesn't compile, but if I remove the 'const' from 
Foo.normalize, then it succeeds:

unittest {
     import std.datetime;
     struct Foo {
         string date;
         DateTime normalize() const {
             import std.format, std.exception;
             int month, day, year;
             enforce(3 == formattedRead(date, "%d/%d/%d", &month, 
&day, &year));
             return DateTime(year, month, day, 0, 0, 0);
         }
     }

     Foo foo = Foo("12/2/2014");
     assert(foo.normalize == DateTime(2014, 12, 2, 0, 0, 0));
}