That's the interesting part from an economics/business school perspective.
If there was a pure low wages vs. high wages argument, then Airbus and Boeing should build their plant in Rwanda. However, on the converse of that why build a plant in the US when the wages are much higher than other countries?
It is about inherent local capability in the workforce for the task required; in short, Airbus needs cheap but literate workers able to learn complex tasks, who are located in an area with ample infrastructure that will not be interrupted for arbitrary reasons. Alabama fulfils that criteria, along with local laws that prevent unionism and encourage foreign direct investment (FDI). Honda, Hyundai, Mercedes and Toyota have already set up there and ThyssenKrupp is building a steel plant in the state, too. The same logic applied to China in the 1980s, there was reliable infrastructure and ample literacy so workers could learn and look where that got them!
Believe me, if Airbus/Boeing just went for an area with the cheapest labour costs, they would spend a lot more building up capability and infrastructure than they would investing in a mature economy like that in Alabama or similar states in the US.
So, it makes perfect sense for Airbus to reduce costs, guarantee a certain quality of product and gain direct access to the NAFTA market.
However, the real competitive question is the C919 and the Irkut MS
-21, will then encroach and takeover the market dominated by the 737 and the A320? I doubt it and have reservations about both projects, but as Arthur C. Clarke once stated, when an established professor states that something is unlikely or impossible, he is usually wrong and so please ignore me on that!