HOW IS IT THAT in the past year and a half, no fewer than 20 black activists in South Africa have died while in police custody? Is there an explanation other than a calculated official policy to physically destroy substantial segments of the country's black leadership, and in so doing to try to intimidate others who would offer South Africa's black majority alternatives to tranquil acceptance of apartheid? Why else would South Africa make a record certain to alienate the civilized world beyond its borders?

The latest victim in South Africa is Steve Biko, 30. He was arrested last month under legislation providing for indefinite detention without trial, and he died this week after (authorities say) a seven-day hunger strike. He was a founder of the South African Student Organization, which represents perhaps the country's most important and volatile political interest group, and a leader of the "black consciousness" movement, which undertakes to build pride and a sense of community among its constituency. A man with an international reputation, he had personal qualities that had established him as one of the likeliest leaders for the government to deal with if it wished to channel rising black rage away from despair and violence.

Few would be surprised if there were to be some sort of public protest in South Africa as a result of Mr. Biko's death. That would no doubt lead the Pretoria government's law-and-order types to identify the dead leader with disruption. But they are the guilty ones. Whether they actually had a hand in Mr. Biko's death is one thing - the suspicion is so strong that only a genuinely independent inquiry, one approved by Mr. Biko's survivors, could conceivably dispel it. In a broader context, those in South Africa who prevent blacks from developing their own leadership corps and who back off from meaningful dialogue with the black community, are sowing the wind. Without the Steve Bikos, they will surely reap the whirlwind.