[Keeper of Dol Guldur]

Tolkien using the term "as a rule" suggests that these sorts of unwritten codes, these usual sorts of things, have over time just made themselves, based on common sense and living peacefully in the Shire.

Everything in the Shire, even the rules, evolved naturally and didn't really need to be enforced or treated overzealously.

We get a great feeling of how the Shire operates.

That contrasts very, very starkly with how the Shire operates during the "Scouring of the Shire", under Saruman's lackeys.

Hobbit's mention "it's the rules" every five minutes, as if they have to repeat these things over and over for them to make sense, as these rules came quickly and from foreign sources, and conflicted with the Hobbit's common sense.

You'll notice Tolkien didn't use 'as a rule' at all once they returned home. It could have been coincidence, but even if it is, you still get the same feeling of restrictedness and discomfort.