Should Consultants bill clients for time they are not paid for?

Computerworld’s Shark Tank had an article today about a consultant that did not bill a client for weekends worked because he was not paid for them.

Shouldn’t it be up to the consultant? If the consultant is giving away his or her time without pay why shouldn’t he or she have the option of creating good will and a good reference for their resume rather than generating a bunch of income for a consulting company to stingy to pay the consultants for work that is billed to the customer?

http://www.computerworld.com/departments/opinions/sharktank/0,4885,109051,00.html