There are unstated rules/norms (and I think they cause unecessary confusion/drama when stating a rule explicitly would reduce some of that).
It is not against the rules to use a running martingale in a hunter over fences class. I can’t imagine a judge using you if you do so, however, because it’s contrary to the unstated rules/norms that you only use a standing martingale.
There’s an unstated rule/norm in dressage (at least the few shows I did where I completely violated this until someone stopped me and kindly and explained it to me) that when you’re warming up before you go into the ring you don’t ride behind the judge’s stand.
It’s been an unstated rule for every type of riding I’ve ever done (and only a stated rule at Devon, nowhere else) that you don’t lunge in the same ring as where people are warming up under saddle. I once did one see someone get asked to leave for doing this and refusing to stop when the steward asked her to stop.
Dressage at Devon had separate warmup areas for stallions/geldings and mares. I don’t think it was an actual USEF/USDF rule that you don’t ride a mare in the stallion warmup area. I think that was a (frankly kind of obvious but still unstated) unstated rule.
I think most shows have some language in the entry that they can refuse and entry or kick an entry out for failing to abide by the rules of the competition.
I’m not an expert on eventing rules, but I am pretty sure the USEA can promulgate rules for events and I am also pretty confident that organizers can set “rules” (whether formal rules or just procedures that must be followed or else they will disqualify you from that event) for their own competition. Maybe “rule” is the wrong word but if a horse trial doesn’t want riders doing XYZ can’t they prohibit that and make it grounds for disqialification? Or, in the case of riding outside the ropes, just rope off any area they don’t want riders going? If it is dangerous to ride outside the ropes areas I’m all for preventing riders from doing that-- but if it’s truly dangerous why not just rope off the area and/or tell riders they will be diqualified if they go around.
I also agree with others that if by going around the ropes DP did put groundpeople/his horse in danger that that could alone be dangerous riding. But my understanding was that there was at least some suspicion that he got the card PURELY for going around the ropes and not for going around the ropes PLUS something else (going around the ropes and thereby coming too close to a person, going around the ropes and onto ungroomed footing, going around the ropes and crossing someone else’s path etc.)
If riders going around the ropes is a safety consideration why leave it to discretion? Why not just rope off all areas you don’t want riders going or tell them they must stay on the prescriped path or else they will face a penalty.