Because they stand up to power:

A Chinese ship carrying arms destined for Zimbabwe was last night forced to turn back after South African unions refused to unload it, claiming that to do so would be “grossly irresponsible”, South African media reported. The reversal is a humiliation for President Mbeki, who had said that the Government was powerless to stop the shipment of three million rounds of AK47 ammunition, 1,500 rocket-propelled grenades and more than 3,000 mortar rounds and mortar tubes to President Mugabe’s armed forces. It was not clear last night where the ship was now destined, or whether it was trying to deliver the arms by a different route. The retreat, if confirmed, would represent a victory for human rights activists, who had filed a legal petition to block the transfer of the goods, and also for the 300,000-strong South African Transport and Allied Workers’ Union, who had said that the arms would worsen the political crisis in Zimbabwe. “Our members employed at Durban container terminal will not unload this cargo, neither will any of our members in the truck-driving sector move this cargo by road,” Randall Howard, a union spokesman, said. “South Africa cannot be seen to be facilitating the flow of weapons into Zimbabwe at a time where there is a political dispute and a volatile situation between Zanu (PF) and the MDC [Movement for Democratic Change],” he said.

Legal action was taken to ensure that the ship would not be unloaded and the South African courts ruled against permitting the ship to be unloaded:

The court ruling followed an application brought with the support of the South African Litigation Centre under legislation prohibiting the supply of arms to "governments that systematically violate, or oppress ... human rights and fundamental freedoms". The director of the centre, Nicole Fritz, said: "It is hard to imagine clearer circumstances in which South African authorities were obliged to refuse to grant any conveyance permit." Helen Zille, the leader of South Africa's opposition party, the Democratic Alliance, warned that the shipment could result in carnage of "genocidal proportions". Pointing out that a consignment of Chinese machetes had prefaced the genocide in Rwanda, she said: "The mind boggles when one considers the damage that could be done with the consignment of arms sitting in Durban harbour."

These arms are still a problem. While the court was making its ruling, the ship left South African waters and its location isn't currently known, although there's speculation it's headed to Mozambique. And even if this shipment of arms doesn't make it to Zimbabwe, things appear to be getting uglier, especially if this report is proven accurate:

Chinese troops have been seen on the streets of Zimbabwe's third largest city, Mutare, according to local witnesses. They were seen patrolling with Zimbabwean soldiers before and during Tuesday's ill-fated general strike called by the opposition Movement for Democratic Change (MDC). Earlier, 10 Chinese soldiers armed with pistols checked in at the city's Holiday Inn along with 70 Zimbabwean troops. One eyewitness, who asked not to be named, said: "We've never seen Chinese soldiers in full regalia on our streets before. The entire delegation took 80 rooms from the hotel, 10 for the Chinese and 70 for Zimbabwean soldiers." Officially, the Chinese were visiting strategic locations such as border posts, key companies and state institutions, he said. But it is unclear why they were patrolling at such a sensitive time. They were supposed to stay five days, but left after three to travel to Masvingo, in the south. China's support for President Mugabe's regime has been highlighted by the arrival in South Africa of a ship carrying a large cache of weapons destined for Zimbabwe's armed forces... According to [the South African newspaper] Beeld, the documentation for the shipment was completed on 1 April, three days after the presidential vote. Zimbabwe and China have close military ties. Three years ago, Mr Mugabe signed extensive trade pacts with the Chinese as part of the "Look East" policy forced on him by his ostracising by Western governments over human rights abuses. The deal gave the Chinese mineral and trade concessions in exchange for economic help.

In previous crack downs Mugabe has used troops on loan from North Korea. China should be asked why it has military advisors in Zimbabwe during a political crisis. Just as they should be pressured over their suppression of Tibet, they should be pressured over their apparent active support of Robert Mugabe.

In fact, it's the kind of pressure the world's most powerful state might be able to exert...if the Bush administration hadn't so devalued our international influence by its disdain for international law and institutions and for the invasion and occupation of Iraq.