The rules in question state they "must be on the sidelines" and the Anthem "must" be played. None have violated that "must". Well, except those players refusing to leave the locker room in protest. Meanwhile, the rest of the rules state they "should" or "may". They "should" stand. Those are not rules, they are options. In any case, it is the choice of the NFL to enforce the rule on fines...or not.