The tone of the letter was surprising to me. I wouldn't expect the FDA to go quite so far in explaining the why behind the desist letter. I suppose that's because it was written just as much for public consumption as it was for 23andMe.
The FDA is a regulatory agency, which implies that their mission is less about intimidation and more about protecting the public good. Hence the more open tone, I suspect.
Its fairly typical of formal regulatory notices (or even informal notices -- much of the language is very similar to the 2010 letter in which the FDA first raised issues with the product) -- because they are typically directed at corporations with substantial legal resources and they are designed to convince them that they should voluntarily comply with the agencies direction because if they fail to do so regulatory action will be initiated which will be costly and unlikely to be reversed by subsequent litigation, so they lay out all the facts and authorities necessary for the recipient (and the recipient's counsel) to come to the desired conclusion.