That person is a problem because low-trust environments are inherently low-privacy and low-efficiency environments. Allowing a small portion of the population to destroy trust and then justifying it with "well there was no explicit rule against it" is parasitic on the whole society. It's better to stand up and say "this is unacceptable and clearly not what was asked for".
That is only as far as you or I are concerned. The environment where you first write the rules then someone can arbitrarily come and say nah that's not what we meant (with any consequences) is far worse than any low trust environment. Vague rules with selective/interpretative enforcement is in fact what authoritarian countries like Russia/China tend to use. Disturbing social harmony is illegal and all the right thinking people know it when they see it.
