Why would something be a "malt beverage" as opposed to a beer? It is still made from barley, but with flavors (natural or artificial) added is my guess. But how would the brewing process differ, and is that even the point in the different identification. I am here talking about things that we would identify as "beer", not malternatives...i.e. Evil Twin "Imperial Biscotti Break", not Smirnoff "Ice".
According to this, it would seem as though all beers are malt beverages, but not all malt beverages are beer. https://en.wikipedia.org/wiki/Malt_beverage
I heard a podcast recently with Randy Mosher, talking about things like the 'Not Your Daddy's Root Beer' and other beverages geared towards the younger folk... ...had I not been listening in a hungover haze, I'd remember more. But it seems like its when a company wants to make something alcoholic that's not really beer but doesn't want to be taxed as if its hard liquor. Like, "lets start in the beer camp to abide by these certain federal regulations and then remove all beer-ocity." That's a real Beavis and Butthead explanation, let me see if I can get you a better one.