ITAR is a US thing...
The "international" bit in the name refers to "international trade" of US firearms/components, rather than "international" or multinational agreement/regulation
So by "pulling out", would mean that the US is scrapping their own set of regulations. May be possible? Who knows, would increase the availability of certain US products, and also hopefully make US shops/manufacturers more open to shipping abroad, as they wouldnt' need to run the risk of falling foul of ITAR (or filling out 20000 forms in triplicate). But equally, their own internal market is fairly strong, so who knows if they'd have any great appetite to expand.