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));
}