It is not a given...this is a worser case scenario...but it is possible. There is no reason in my mind why an established couple would be 'silly' to ensure some rules were in place to mitigate that risk.
Similarly I would not consider it silly for a prospective third to want rules to mitigate risks to them....since couples are twice as capable as behaving badly as a single.
The established couple may have rules that apply to THEIR relationship, but they shouldn't have rules that pre-emptively dictate how their OTHER relationship(s) unfold and evolve.
The same thing re: the "prospective third". "They" may want to have rules about the relationship(s) THEY are in, but they don't get to dictate, "You two need to be / cannot be [a certain way] when you're together so that I can feel secure."