It can very well be a principle of law. In certain legal codes, it actually is.

As for reality, that's a pretty low bar.

We have many "shoulds" (e.g. you should not cross a red light lest you get a ticket / prison) that are not some inherent property of reality or physical law, but things we've decided upon.

In fact, those are the only things that makes sense to consider as "shoulds". The constraints of reality are not negotiable, so should there is superfluous.