There is, apparently, an article in the Winter 1995 issue of Zymurgy by Dennis Davison that covers this. I don't have access to the article and have not read it (and of course it is fairly dated at this point), but here is a quote I found on another forum. I make no claim as to its accuracy. Tracking down the article might be a good starting place.
"According to Bureauf of Alcohol, Tobacco and Firearms officials, the process of freezing beer and removing ice is called concentrating. A brewer may not employ any process of concentration that separates alcohol spirits from any
fermented beverage, and since ice is being removed from beer, this concentration procedure is legal."