First, having the candidate provide the type of work-related references asked for by the prospective employer practically guarantees candidates will, if they haven’t already done so, ask the appropriate people to serve as references before submitting them to a prospective employer. In other words, they will have gotten permission to list these people as references. Furthermore, the mere act of asking dramatically increases the likelihood that references will talk about the candidate to a prospective employer—because, more than likely, they will be expecting the call.
By the same token, if they’re not willing to answer questions about the candidate, why would they have agreed to serve as a reference in the first place? More importantly, if they decline to discuss the candidate for whom they’re agreed to serve as a reference, a major red flag should immediately go up in the mind of the prospective employer. Taken from another perspective (that of the job seeker), why would you ever list someone as a reference without asking the person first or without finding out up front if he or she will be willing to talk to a prospective employer when called?