There is an old proverb that "good fences make good neighbors."
Benjamin Franklin also once said to "love your neighbor, but don't yank out your hedge."
Who Said "Good Fences Make Good Neighbors"?
Our sense of "space" (or territory) defines us as a species from the beginning of our awareness of self. While homo-sapiens are social beings, they are fiercely territorial. Some branches of our tree are comfortable with less (like the Japanese for example), and some require more. Folks who live in the wide open spaces of the American West know what I am talking about when jammed into an elevator in New York.
This concept of defending "our space" leads directly to our hard wired need to defend our property as well (since by definition they are IN our space).
The opening scene from the movie "2001: A Space Odyssey" depicts the entire concept very well. No amount of social engineering will eliminate the need for personal space and no amount of wishful thinking will eliminate our need to defend "our stuff". RULES (laws) are made by humans to allow larger groups of humans to coexist peacefully.
When aggression against the rule of law, (and we rule keepers), becomes common place, the breakdown of who we aspire to be becomes blurred by who we were from the dawn of time.
The "rule" about cutting through another's campsite is unwritten, but is based upon the respect of one family group's space as you would want that family group to respect yours.
I am not surprised the confrontation became heated, because defense against aggression is a natural response. Just be glad you did not need to use a jawbone to send them packing!