This should not be really surprising to those who have read for instance George Catlin's diary, who reports seasonal use of green maize as complement to an otherwise hunter diet among the Mandan (a people from further north anyhow). Some peoples in the early stages of transition to agriculture may have adopted only certain, more profitable aspects of the new technology, making a slow transition.
William R. Merrill et al., The diffusion of maize to the southwestern United States and its impact. PNAS 2009. Not open access yet, news article at Science Daily anyhow.
The authors conclude that the American staple crop was passed from forager group to forager group through the Southwest of the USA (Texas, etc.) and has no relation with the spread of Uto-Aztecan languages, which in fact pre-dates agriculture and was a north to south migration. SW peoples also show a gradual independent transition to making pottery.