A sub asked me the same question some time back as someone wanting to become her Master had forbid her talking to others. My reply was that such rules were not ones that I would use. I did not say so at the time but my belief was that it was very much a sign of insecurity. It may also indicate a lack of experience, after all I believe most experienced Masters are comfortable with their position and know that a sub serves them through love and respect.

Obviously each Master/Dom may wish to set individual limits, as others have mentioned above, but to forbid any contact with others seems draconian.