This effect is a function of political legitimacy. The modern era is heavily informed by the principles of the Liberal Enlightenment, and in Enlightenment terms political legitimacy is conferred by the appearance (real or constructed) of popular support among citizens. This means that even the most brutal and dictatorial leader must weigh the impact of his actions on the public consciousness, finding some way to legitimize his actions as matter of public good.
Opposition leaders, public intellectuals, popular artists, and the like have broad public followings of their own. Extreme attacks on such people — beatings, torture, 'disappearing' them — can produce broad public backlash that threatens the legitimacy of the regime. Such acts look far too much like the regime leader exercising a personal, vindictive program than upholding the 'rule of law' in the nation. It's usually easier and wiser (from the regime's perspective) to allow such people a certain latitude, finding small and justifiable ways to delegitimize them: convictions on minor crimes, scandals of one sort to another, disputing their patriotism, intelligence, mental health, etc... Even a brutal modern dictator must appear as though he respects the will of the people, and if an opposing public voice has currency in the general public, that currency calls for discretion from the regime.
More or less faceless citizens, by contrast, have no such protection. The regime can present them as violent insurgents, criminals, or thugs, and can indulge in any level of outright brutality that it can pass off as necessary for securing the public good. In some cases this backfires, when a particular cause attracts the attention of the general population without an express leader — e.g., the Black Lives Matter movement in the US, and the snowballing reactions against police efforts to suppress those protests through militarized means — but in general brutality against faceless citizens has no immediate cost for a regime, and has positive value in terrorizing non-compliant portions of the citizenry into submission.. SO long as the leader can stand up the next day and say "See, I protected out nation from ...", his political legitimacy and the political legitimacy of the regime will be unscathed.