Why are people desperate to defend NAFTA?

Inexplicably, Reason repeatedly prints bad arguments in defense of NAFTA. This time it’s the Chicago Tribune‘s Steve Chapman saying that NAFTA boosted workers’ earnings:

Ordinary workers, contrary to myth, benefited from NAFTA. In the decade before it took effect, according to the Bureau of Labor Statistics, average hourly earnings (adjusted for inflation) fell by 5 percent. In the decade after, they rose by 10 percent.

Nonsense. You’d have to control for other factors to make such a comparison meaningful. Such as 1990s labor productivity growth, which was “the defining economic event of the past decade” (pdf).

While Chapman’s piece does contain some respectable economic reasoning, the inclusion of such a glaringly bad argument undermines its credibility. Why are the folks at Reason so eager to defend NAFTA?