The most common sort of cease and desist letters we tech people are accustomed to are DMCA and patent-related. These letters are typically meant as precursors to private litigation. They are often sent out with commercial considerations in mind, and so tend to be light on details in order to be more threatening.

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.