When I was at the FAA, we lawyers received regular reminders that we were not the policy makers. We were supposed to figure out what the law says, what the boundaries might be of what the law allowed or required, and otherwise act as analysts of the law. In doing so, we were not to replace whatever the law said with what we thought was the best policy. We were just supposed to interpret the law.
A lot of people–some lawyers and government bureaucrats included–think law and policy are the same thing. They’re not. When people talk about policy, they’re talking about ideas regarding technology, ethics, economics, or a host of other issues. If something is good policy, it’s a good idea. It is a good idea to brush your teeth every day, but it is not the law. It could be a good policy. Conversely, Congress could enact a perfectly clear law that was bad policy. For example, if Congress were to require federal oversight of every activity ever by every American in outer space, that would be pretty clear but not necessarily a good idea. It is my own policy view that such a law would stifle innovation, burden private actors with the unbearable tightness of regulation, and delay, delay, delay all the good that could come from humanity venturing into space. However, if such a law were to pass, my policy objections to it would have no bearing on whether or not it was mandatory. It would be, because it would be the law. The law is mandatory.
Some laws are vague and provide a lot of latitude in interpreting them. Then the policy makers can exercise their policy muscles, but they must remain within the confines of the law itself. If Congress gave your agency the task of regulating reentry and told you to define “reentry” as the return of a vehicle from outer space, you don’t get to use that authority to regulate former felons reentering society, even if you have a really, really, super good helpful idea. Policy views, no matter how well intended, don’t change the law.
Under the Constitution, Congress enacts policy–its ideas about what would be good to require– into law. The regulatory agencies of the Executive branch then carry out that law–they “execute” the will of Congress. When Congress is clear, the Executive branch is supposed to do what Congress tells it even if it disagrees. When Congress is vague, which happens more often than it should, the Executive branch has room to develop policy views of its own and then make them mandatory through regulation.
Hoping this helps.