well then if youre going to go with that, then shouldnt parasites and viruses be able to infect only creatures on their side of the fence? or perhaps the most applicable comparison would be otys, they dont just eat creatures on their side of the fence.
interestingly, barbed wire was invented to prevent free-grazers from having their herds eat off of other peoples lands as they moved the beef cross country to sell (while the beef was still living). so if someone made a barbed wire fence card (which element would that be, anyway?) then that would limit MooMoose to his own rustlers.