When NAFTA was negotiated Mexico was not controlled by drug cartels and crime bosses. The borderlands near the US are essentially lawless states that export drugs, weapons and criminals to the US, along with some goods covered by NAFTA. That isn't Canada's problem, nor should it be. 
Mexico has ceded it's sovereignty to drug lords, until an accountable government can retake control, Mexico should be left out of NAFTA, or at least be treated as a separate entity.