I'm pretty sure it's a label requirement for the USA (or certain states, as mentioned above). I'm not 100% sure of the regulations, but I don't think the TTB distinguishes between "ale" and "lager".. I think all "beer" is labeled as ALE. We have a few lagers at the brewery I work at that also say ALE on the label.. I've always wondered, but I think it's label requirement. I wouldn't be surprised if it dates back to when all the breweries in the US were German style lager brewers and then people started making ales and they had to mark it in such a way to say that "Yes, this is beer" and then it stuck.
I'm pretty sure the ALE distinction is to separate it from the other malt beverages, such as Malt Liquor, Flavored Malt Beverage etc. It helps separate things like beer from Seagrams Escapes and Mikes Hard. Different tax codes.