Here are more details of how doctors (and dentists) in Ontario won the right to income-sprinkle among family members. 
The right of these professionals to incorporate resulted from 2001 legislation under the Harris government.  However, because of concerns that non-professional shareholders could undermine regulatory standards, it was decided that only members of the profession could be shareholders.
Subsequently in 2005 as part of a labour agreement with Ontario doctors (worth $2.4B over 4 years), the Province agreed to allow families of doctors to become shareholders which enabled income-sprinkling to family members through dividend payments. At the request of the Dental Association, this right, which went into effect Jan 1, 2006, was also extended to dentists. Since then other professions have lobbied, so far unsuccessfully, for similar treatment.  
See: http://www.advisor.ca/special-reports/incorporating-differences-26501