In case you haven't heard, public gatherings have been outlawed in Zimbabwe for awhile. One of the tactics used by brutal dictators to stay in power is to severely limit personal freedoms. Anything seen as a threat to their power: newspapers, political parties, unions, etc. are always outlawed. Besides this recent arrest, opposition leader Tsvangarai has been charged with treason twice in the past five years and even been accused of plotting to kill Mugabe.
In a country where the unemployment rate is at 80% and people struggle to buy the most basic of life's necessities, people are expected to suffer silently. For now, Mugabe has the support of the police and army probably because he is paying them in British pounds or US dollars and not that worthless Zimbabwean money. As Zimbabwe falls apart, South Africa remains silent.