> There is no moral principle in any system anywhere in the history of the universe that requires me to bind myself to a contract that nobody else is bound to.

Is there not? I don't feel this makes sense to me, as the conclusion seems to be "if everyone (or perhaps a large amount of people) do it, then it's not immoral". My immediate thought goes to moral systems that universalise an action, such that if everyone did it and it makes the world worse, then it's something that you should not do. That would be an example of a system that goes counter to what you say. Since morals are personal, you can still have that conclusion even if other people do not subscribe to the same set of moral beliefs that you have. Something can be immoral to you, and you will refuse to do it even if everyone else does.

> But if nobody ever agreed to it [...] there is no reasonable ethical principle you could propose that would prevent me from grazing all my cattle too.

Why not? I don't quite understand your conclusion. Why could the conclusion not be "I feel what everyone else is doing is wrong, and I will not do it myself"? Is it because it puts you at a disadvantage, and you believe that is unfair? Perhaps this is the "reasonable" aspect?

Your confusion is understandable. The way the terms "moral" and "ethical" are thrown around is sloppy in most vernacular. Generally, ethics refers to system-wide morality. E.g., I may feel that personal morality compels me to offer lower rates to clients, even though a higher rate may be acceptable under legal ethics. I tried to make that distinction clear in my post ("moral principle in any system") but perhaps I didn't do a good enough job.

The original poster was not referring to individual moral feelings, but to formal ethical systems subject to systematized logical thinking: "classic example of an ethically unsound argument."

There is no religious tradition, no system of ethics, no school of thought in moral philosophy, that is consistent with that position. The closest you might come is Aristotelian virtue ethics. But it would be a really strained reading that would result in the position that opting out of commons mismanagement is required. Aristotle specifically said that being a fool is not a virtue. If anything, a virtue ethics lens would compel someone to try to establish formal community rules to prevent the tragedy of the commons.