Yeah, why are people against NAFTA? I truly don't understand that.
The most solid ground to attack it is the Mexican farmer suicides. Which is a direct responsibility of the protectionism the U.S. encourages with its outrageous corn farm subsidies. (And yes, I know the EU is just as much a villain in this respect)
Luckily, the TPP seems to undercut the Japanese corporate agribusiness a bit, which is very good because they are a terrible racket and quite surprising seeing as the LDP are in power at the moment.
Well that's grounds to accuse NAFTA of providing for insufficiently free trade, not the opposite.
I've actually seen people on the right who, at least theoretically, support free trade but oppose NAFTA for that very reason.