The reason man-bites-dog is news is that it rarely happens. That's also why Cabinet member-bites-president makes headlines: It, too, hardly ever occurs.

Under the U.S. presidential system, Cabinet members are merely hired hands, serving at the pleasure or whim of the chief executive. Few have constituencies of their own, unlike the ministers of parliamentary democracies, who often cannot be dismissed without inviting a political crisis.

Hence, in looking back over this century, it is not surprising to discover how very few U.S. Cabinet officials have dared to challenge the president publicy. The record indicates that Andrew Young, our ambassador to the United Nations, is the only one who has repeatedly done so. Most of the others were fired before they could repeat lese majesty.

It may be that Young would have been ousted, too, had he not a strong and loyal following of black voters, but, beyond that, he has been fortified by the rush of events that have vindicated many of the independent positions the outspoken ambassador has taken in his brief diplomatic career.

It's risky to fire an official for being right on major policy, even if he is verbally reckless at times, and at the moment Young is basking in worldwide acclaim for the latest and most notable achievement of the African policy he has steadfastly promoted, often in the face of opposition from the White House hawks.

Only a few weeks ago, Carter's cold warriors were exploring the possibility of new covert operations in Angola, with the intent of undermining the government of Agostinho Neto, which came to power three years ago with the help of Cuban troops.

Andy Young, first to last, has maintained that Neto, although supposedly Marxist, would gladly cooperate with the United States if given a chance. His judgement was affirmed when the administration, backing off from intervention, joined in a diplomatic mission to Angola, which, with Neto's active help, won black acceptance of a Western plan for the independence of South-West Africa (now known as Namibia), which adjoins Angola.

The problem was to get a stubborn and suspicious SWAPO (Namibia's guerrilla liberation force) to go along on a program, already accepted by South Africa, for peaceful transition to independence under U.N. supervision.

All the Western negotiators agree that Young, who conceived the Namibia strategy, paved the way for the breakthrough by improving the negotiating atmosphere and restoring U.S. credibility in Africa. Angola's cooperation, said one official, "was absolutely essential" to getting the agreement.

Some of those now calling for the dismissal of Young were last year denouncing him for saying there was a sense in which the Cubans brought "a certain stability and order" to Angola. Was he far wrong? The Cuban presence has enabled Neto to maintain a viable government, protect U.S. investments like the big Gulf Oil Company operation at Cabinda and move toward a good-neighbor policy with next-door Zaire.

Although Young himself has long been a crusader for human and civil rights, he had doubts from the beinning about President Carter personally attacking Russia on that score. Well over a year ago, he publicly said it was time to "let up" on the criticism.

His concern then, as now, was that the criticism would boomerang, with Russian dissidents ending up worse off than before. He was also concerned that official U.S. moralizing would not be received well in most other countries, especially in light of America's own long history of abridging human rights.

Compare the fate of Alexander Solzhenitsyn, the most infuential of all Russian dissidents, with that of a relatively minor dissident like Anatoly Scharansky. During the Henry Kissinger era of detente and quiet diplomacy, Moscow allowed Solzhenitsyn to leave Russia, become a millionaire and the world's leading propagandist against the Soviet system. Today, Scharansky and even lesser dissidents are getting harsh, long-term prison sentences and banishment to Siberia.

Young says Moscow is making a "terrible mistake" in trying to stifle the human-rights movement, but senators and others are now calling for his dismissal because he also noted, again in careless extravagant terms, that his own country has some things to account for in that respect.

Instead of loosely talking about hundreds or thousands of "political prisoners" in the United States, the ambassador would have been on sounder ground if he had cited the American record on slavery, the denial of suffrage to women until 1920, the long suppression of the labor movement by massacres, injunctions and jailings, and the shootings and arrests during the civil-rights movement.

Young could also have called attention to the FBI persecution of the late Martin Luther King Jr. the government's prosecution of Vietnam dissidents like Dr. Benjamin Spock, and the civil-rights depredations of the undercover "plumbers" in the Nixon White House. Also, more than 1,000 American Indians have been marching around Washington this week protesting government violations of their rights.

Certainly the American record has improved over the years. Young thinks there could be gradual improvement in Russia, too, if detente is restored. "Considering that Russia is a police state," says Time, "the dissidents have been allowed remarkable opportunities to protest and publicize their grievances." In Stalin's day, they would all have been executed long ago.