My old law partner liked to harp on this point as well, and it is a valid one. However, as a matter of practice most well-drafted work-for-hire agreements also

My old law partner liked to harp on this point as well, and it is a valid one. However, as a matter of practice most well-drafted work-for-hire agreements also contain a backup assignment clause something like "to the extent the work is not a work-for-hire, I hereby assign the copyright". "Work-for-hire" is one of the many misnomers you find in commercial contracts, along with "intellectual property".