Treasury Board authorizes the use of the social insurance number under the directive of the use and operation of the social insurance number. Departments can obtain that authority either through the Treasury Board through regulation or they can do it through the departmental legislation. It is not for each department. Departments have to demonstrate a need to collect it, and what they would want to collect the SIN for. Given that the SIN, and more importantly the social insurance register, holds a lot of sensitive personal information about Canadians, only those departments which can demonstrate a need for it collect it. There is a process to go through to determine which departments and programs would like to collect the social insurance number and for what purpose.
On October 28th, 2014. See this statement in context.