FAA Chief Legal Counsel Letters of Interpretation are not an example of cowboy actions. That someone might think so exposes a gross ignorance of the regulatory process and the administration of the regulation.
There are three primary means of understanding the regulation: the first are the Federal Register preambles; the second are FAA Chief Legal Counsel letters of interpretation. The third is the regulation itself.
A fourth means, which does not interpret the regulation or set precedent, are ALJ renderings regarding specific cases; these do not define or interpret the regulation, but do give some insight into how it has been applied in specific cases in the past. Such cases do not set precedent for future application of the regulation.
Chief Legal Counsel letters of interpretation are not examples of the FAA "making it up as they go." The FAA Administrator is charged with regulating, promoting, and enforcing aviation, and the Administrator has authorized the Chief Legal Counsel to interpret the regulation. Chief Legal Counsel interpretations are defensible and are a legitimate means of interpreting and understanding, as well as applying the regulation.
The FSDO level does not have authority to interpret the regulation. The FAA Chief Legal Counsel and in specific cases, regional legal counsels) do have authority to interpret the regulation, and these interpretations are searchable and represent a legitimate, authorized interpretation that an airman, operator, or the FAA may hold up as official, bona fide interpretations.
The FAA does not "send them out" and they are not published in the federal register, nor are they found codified in the regulation. The FAA Chief Legal Counsel interpretations are just that: interpretations of the regulation. While the Federal Register preambles will provide the rational and intent of the regulation and the regulation itself will provide the codification of the rule, the Chief Legal Counsel interpretation will explain in detail answers to specific questions by parties of interest on given subjects applicable to the regulation (such as the prospective nature of rest, for example, or questions regarding Part 61.51, logging of flight time, etc).