NAFTA was a lousy deal. RayGun's negotiator's kept stringing LBs reps along until Mulroney had invested all his political capital. Then the USA legislature wanted it made even more unbalanced in favor of the USA. Look at the Softwood Lumber chronic headache. We can't even ban cancerous fuel additives such as MTBE that are banned in the USA without threats of being taken to court under NAFTA. 
https://en.wikipedia.org/wiki/MTBE_controversy