Because Captain Pike decided, for better or worse, to give them a break.
As he is portrayed in the Abrams-era films, Captain Pike is a firm but nice guy. If "Cupcake" (as he has come to be known) and the other cadets had a note in their records saying that they beat an unarmed civilian to a pulp for no apparent reason, they might have a tough time finding future assignments, and that's assuming they aren't thrown out of the Academy itself.
Pike gave them a second chance. This is a luxury that Pike also grants Kirk several times in both Abrams-era films.
Note: I'm not saying that Pike should have given them a second chance, nor am I saying that he has the authority to do so. He was the only senior officer present in the bar and chose not to report the incident to the cadets' superiors at the Academy. If someone can face disciplinary action for cheating on a test (i.e. Kirk), they would most certainly face disciplinary action for beating an innocent unarmed man senseless. Since their careers have suffered no ill effects — Cupcake was an officer on the Enterprise in Into Darkness — we must assume that Pike did not report the beating. By doing so, he subverted the normal course of disciplinary action. Whether that is right or wrong in this case is beyond the scope of the original question.