The US Department of Justice is not the judicial arm of government, but the executive.

It is often not possible or desirable to have laws that are so complete and exhaustive that they require 0 interpretation. Laws, like most thing, are designed to try to balance flexibility and clarity where necessary. Otherwise, they are mostly worthless, or become worthless very quickly. (and no, you can't just make them super explicit and constantly update them, it's completely intractable)

As a result, pieces of government offering guidance/manuals for their enforcement is very common.

This is true both criminally and civilly.

For example, the USPTO maintains the "manual of patent examining procedure" that somewhat exhaustively interprets patent law.

SCOTUS already shrunk the scope of some laws (I think it was the CFAA) where they disagreed that simply violating a local policy about computer usage === CFAA.

I think this is a slow but natural process to narrowing it down.

Note this is the executive branch not the judicial. Sadly laws are so hard to legislate now this is how fixes are often being done - piece meal, weakly, and subject to random changes by political whim.

Checks and balances. It's not great but it is a way around the current legislature which has become increasingly paralyzed by partisanship.

It's one of the many checks and balances we have available.