THE MAGISTRATE'S verdict exonerating the police in the death of Steve Biko, leader of the black-consciousness movement in South Africa, is an indictment of South Africa. The ruling is just as "inconceivable" as the State Department said it was. The magistrate found that Mr. Biko had died of head injuries incurred in a struggle with police, but that the evidence "does not prove the death was brought about by any act or omission involving an offense by any person." Not "by any person"? Mr. Biko had been kept naked and shackled and, despite conspicuous symptoms of distress, given no medical treatment. A police officer, saying Mr. Biko was "shamming," refused to have him taken to a local hospital. Only when he was found foaming was he taken - driven naked 700 miles in a Land Rover - to the prison hospital in Pretoria where, unattended, he died.

Mr. Biko's death was at least the 20th in 20 months of a South African black being held under "detention," that is, under special legislation that suspends the protections accorded citizens arrested under the regular criminal code. The number of people currently known to be in detention is 714. There was no way for Mr. Biko's family to penetrate the self-contained detention system to bring out the truth about his death. The inquest that the family could and did demand, however, was held within the other, or open, part of the South African legal system. Thus the outside world could get some glimpse of the atrocities that whites in power commit against those they choose to see as "cheeky" blacks like Steve Biko.

One must be thankful that such glimpses are still permitted. There is a real possiblility that such inquests will be shut off in order to shield from public view a repressive policy that many South Africans believe will probably be intensified. Enhancing this possibility is South Africa's awareness that the international community is now focusing sharp attention on the crudities of apartheid. Such a change would take South Africa a long way away from the legal procedures on which is has relied heavily up to now to claim a close kinship to the West. The South Africans who are pushing their country toward a Soviet-style police state bear a heavy responsibility.

When will white South Africa come to see its own interest in change? Broadly, two developments are necessary. The first is to expand, rather than contract, the reach of law so as to end police brutality as an instrument of repression and intimidation against blacks. It was not necessary to kill Steve Biko and the other detainees. People who claim to be Westerners should be able to deal with real offenses against law and order within a single legal system. The second thing is, of course, for white South Africa to diminish or moot disputes over law enforcement by moving on to consult the non-white majority on their grievances. Steve Biko was the kind of black leader - prepared to negotiate with whites, able to lead a substantial part of the black community - with whom any sensible white would have wanted to consult. Instead he was murdered, and now his murder is covered up.