One of the most minimal criteria for the legitimacy of a regime is that punitive power is exercised in accordance with the rule of law. The rule of law requires that like cases are treated alike, so that, for example, two people who commit the same wrong are, as long as they are relevantly similarly situated, punished in the same way. Obviously, cashing out exactly what is meant by ‘relevantly similarly situated’ there is difficult, but we have a fairly firm grasp on at least a core of cases where miscreants are ‘relevantly similarly situated’: the colour of someone’s skin is not a relevant difference, and neither is whether or not they are related to or a friend of the person administering the punishment. The rule of law, as a concept, basically rules out the arbitrary exercise of power.
Notice that the rule of law doesn’t just rule out treating relevantly similar cases differently. It also requires that punishments are proportional to the wrong in question. We would think that a system of punitive power that admitted that act a was worse than act b but punished act a less seriously than act b failed in exactly the same way that a system of punitive power which treated acts c and d, which were relevantly similar, differently. It would be arbitrary in exactly the same way, since it would fail to give attention to the proper criteria for punishment. If it’s wrong to punish a black person who steals more harshly than a white person, then it is also wrong to punish a murderer less severely than a thief.
Consider for example these two cases. In the first, a group of twenty or thirty students hold a loud impromptu party well into the night in a staircase in student accommodation. On several occasions, other residents of the staircase complain, as they are being kept awake, but to no avail. In the end, after a number of hours, the appropriate authorities are contacted, and the party broken up. In the second, a student leaves their room for less than an hour in the early evening, leaving their radio on behind a locked door. A neighbour complains of the noise, and the appropriate authorities come, open the door, switch off the radio and leave.
One would imagine that the first case would be regarded as much more serious than the second. An authority which treated the second as worse than the first would be an obvious example of an arbitrary authority, an authority whose legitimacy was seriously in question. An authority whose legitimacy is in question should be challenged. Obviously, there are some challenges which are ruled out of court. It would be illegitimate to embark on a campaign of terrorist bombings in response to one case of disproportionate punishment. You might say that there was a requirement of proportionality that applied to challenges to arbitrary authority, that challenges to authority need to meet standards similar to those which authority itself must meet. Certain kinds of challenges may well be gratuitously offensive. However, whilst certain people with internet access are surely 'unpleasant', they don't have the power to force people to spend several hours picking cigarette ends from the gaps between flagstones. This view clearly has wider implications.