The nuance here is that they're using an unconventional definition for "employee" and "independent contractor". Under the heading "Supreme Court Interpretation":
> If an employee created the work, part 1 of the definition
> above applies, and the work will generally be considered a
> work made for hire.
> But note that the term “employee” in the definition differs
> from the common understanding of the term. For copyright
> purposes, “employee” means an employee under the general
> common law of agency. See the subheading “Agency Law”
> below.
> If an independent contractor created the work, and the
> work was “specially ordered or commissioned,” part 2 of the
> definition above applies. An “independent contractor” is
> someone who is not an employee under the general common
> law of agency.
That is, for purposes of determining whether or not a work was "made for hire", an independent contractor is considered an "employee" under the "common law of agency", because he is acting as an agent on behalf of an employer.
This is discussed further under the heading "Agency Law", where three points are given for consideration of whether the contractor is acting as an "agent" of the employer or whether the contractor is acting independently.
The phrasing of the law probably would've been better if it said "external vendor" instead of "independent contractor", since it seems intended to protect vendors that are providing a simple product to the employer rather than contractors that are used like temporary employees.
Regardless, a copyright assignment is a simple and important way to clear any ambiguity and prevent hairy disputes over the status of a work from going to court.
Again, I am not a lawyer.