There's no reason to follow the internal logic of a religion when trying to understand its workings. With Sabbath, observe how the rabbis decide on how to apply the rules. They know that their task is to keep the society functional, even if they use religious language to express it.

One basic reason a community will have some arbitrary rules: They make it expensive to be part of the community. They make it harder to fake adherence and profit from the group. All members of the community understand this intuitively even if they can't express it. So they welcome what looks like silly rules to the outsider.