Can you say why you think it matters? What - to you - matters about the distinction?
Are you familiar with the card game "Mao"? It's a game explicitly and intentionally built around only having unwritten (or, if you prefer, unspoken) rules. Some versions even have the rules "text" (allowing "text" to include spoken words): "The only rule you may be told is this one." Some explain the basic premise (to win, you must discard all of your cards), some literally won't say anything at all. Play progresses by players being told (by other experienced players, or the "Chairman" player who enforces penalties)
which of their actions broke a rule, but not what the rule was; all rule violations result in needing to draw another card.
Mao can be extremely frustrating to play if you aren't
really prepared for it, and is
critically dependent on the "Chairman" being genuinely thorough and fully consistent. A "Chairman" who is even mildly inconsistent will make it nearly impossible to actually learn the rules via observation, experimentation, and logic.
I bring this up because Mao shows what it's like when you have "rules" that are inherently tacit. It shapes every consideration of how you play. And it directly leads to most of the issues players are likely to run into. It's also a game where introducing "Rule Zero" as an official rule would completely destroy the ability to
play the game at all--and yet "change what the rules are" is an inherent part of making Mao an interesting game. Because if there were only one single set of rules, Mao would be a very quickly solved game, and people would lose interest. Instead, there are many, many variations of Mao, and people may change which variation (or implementation thereof) they use
between sessions. Once
in a session, however, Rule Zero would be a poison pill, killing the gameplay entirely.
To implement Rule Zero would be to make Mao impossible--because the rules
could not be deduced through experimentation and logic. Yet, to entirely deny the ability of players to change the rules of Mao would make the game pointless, because there would be no need--just look up the rules and you're done. Only by being a game where "rules can be changed" is a player behavior, but Rule Zero is completely
absent, can you get a Mao that is actually worth playing.
As a secondary question, do you have in mind a definition of rules, principles, and behaviour, that you are using to know that it is the latter? Can you say something about that definition?
Granting that some things can be a little fuzzy, I would consider them as follows.
Rules: Specific instructions for doing particular things. E.g., Discern Realities is a set of "small," focused rules which give specific instructions to the players. Sometimes, collections of such "small" rules can be referred to as "mechanics."
Principles: General concepts or methods to employ. E.g., "draw maps, leave blanks" is intentionally
not specific--"maps" and "blanks" refer metaphorically to the whole process of GMs introducing content into the game. Principles can still heavily shape gameplay, but they do so in more qualitative, descriptive terms.
Behavior: Whatever it is players (counting the GM as a player) do. I apologize for it being an almost uselessly broad definition, but "behavior" is almost a primitive concept, something you just have to accept like the definition of "point" in geometry. In theory, "behavior" precedes either of the previous, but it can also be spurred on by one or both too.
Perhaps a useful way to think of it would be that "rules" are like specific codes of behavior (e.g. "thou shalt not bear false witness"), while principles are Aristotelian virtues, which are normative by way of telling you what star to sail toward, not how to steer the ship or tie the knots or tack into the wind. Both have behavior-shaping force, but in supremely different ways.
The issue, then, with the comparison between "rule zero" (whether it is explicit or implicit) and "players of games change the games they play," is that the former presents itself as creating/allowing/establishing a thing, as such creation/allowance/establishment as distinctly second and optional for people playing games. The latter is purely descriptive, "That's a thing people that play games do." And from that distinction, of entrenching vs describing, much of my concern flows.
It would be like someone talking about the rules of group writing, and someone saying "yes but the Lead Author has the special right to change what is actually written, which no one else has and which is not naturally part of writing. This makes Lead Author-driven writing specially flexible." The declaration implies that, without this "writing Rule Zero," no one would have the ability to actually edit the written work...when being able to edit what you're writing before you commit to it (outside of things like words carved on rock!) is simply an inherent
behavior of people-who-write.
Chekhov's Gun is a writing rule, summarizable as, "Don't put drama-inducing tools into your story if you aren't going to actually
use them to induce drama." "Show don't tell," on the other hand, is a writing Principle, since it isn't anywhere near specific enough to implement in itself, but rather a goal to strive toward in appropriate contexts. (Sometimes, telling is better than showing! But
generally it's better to show, and you should use telling only when it is helpful, not as the default state.) "Change what you've written" is a behavior
literally everyone who writes engages in.