We have a job recruitment system that we installed which includes a public website where people can apply for jobs directly on the internet. One of the fields is SIN number that displays by default. The vendor is not willing to mark it as optional. Their explanation is since all required fields are marked "required" under the text boxes, it implies the rest of the fields are optional and they don't have to mark SIN field exclusively as optional which would confuse the applicants. They have a point but I am concerned.

Is the vendor's explanation correct?