The reality of NAFTA regarding food such as produce and meats is that these products are not labeled.
That may be so, but that doesn't mean that the food can legally be contaminated. The agreement requires that all food meet international requirements for safety and sanitation. At the end of the day, that's all you can do. If some farmers in Mexico don't follow the rules, it's no different than some farmers in the US not following the rules.
The fact remains that under the NAFTA, the US has the power to block imports of food and agricultural products that don't meet international standards.
So what? What does a label have to do with the fact that something might be contaminated?
And in the case of the infamous Californian/Mexican strawberries of the 90's, it was a U.S. company that fraudulently represented Mexican strawberries as home-grown in order to qualify for school food programs. There ain't no trade agreement that will prevent that sort of activity.