http://www.gsa.gov/portal/forms/download/117242
It appears that only Departmental Regulation to sign the form, but not any law, was violated.
I may be, and often am, wrong, though.
I was a little confused about what the optional meant too, but I think it means that the GSA doesn’t require it of all departments, but if State requires it, you must sign. According to the info I found the State Department requires the form to be signed and returned when the personal ID is returned.