Placement agencies can avoid responsibility for source deductions by placing incorporated workers

A placement or employment agency need not deduct and remit source deductions and employer premiums if payments are made to a corporation (such as an incorporated worker) for services rendered.  Thus, a placement agency may limit its potential exposure for such deductions and premiums by placing incorporated rather than unincorporated workers.

Neal Armstrong.  Summary of Michael Gemmiti, "Placement Agencies: Insurable and Pensionable Employment", Canadian Tax Highlights", Vol. 22, No. 3, March 2014, p. 10 under s. 125(7) – personal services business.