TALK ABOUT inconsistent treatment of demonstrators in this capital of the free world:

Some 1,655 people are charged with breaking a law by demonstrating within 500 feet of the South African Embassy and U.S. Attorney Joseph E. diGenova has all federal charges dropped before anybody even goes to court. Three other people do the same thing in front of the British Embassy and they, too, go free with charges dropped. Then one woman is arrested for the same offense in front of the Soviet Embassy -- and Mr. diGenova proceeds to prosecute her. Can you explain the difference?

If Mr. diGenova can, he should -- because neither the public nor the woman, Vanna Om Strinko, a refugee from Cambodia who is now a U.S. citizen, has heard any sensible explanation for this uneven justice. Prosecutors do have almost unlimited latitude in deciding which cases they want to take to court, but there's no reason why they should not explain, to an understandably curious public, what it is they think they're doing. If 1,655 people (at last count) are let go, why not 1,656? Or why not take all the cases to court for disposition?

The Washington Legal Foundation, which is representing Mrs. Strinko, perceives a difference in the treatment of "liberal" and "conservative" causes. If there is anything to this, it should be clear all around that such an approach has no place in a fair system of justice.

The political aspects of this contrast become doubly fuzzy given still other differences in the treatment the protesters have sought and in the handling of cases at the federal and local levels. The leaders of the demonstration at the South African Embassy actually have wanted their day in court, with all the public exposure and whatever punishment that might come from it. In some of the cases that came under local jurisdiction, the D.C. Corporation Counsel cited the merits of the demonstrators' views as grounds for not prosecuting. That's totally wrong, and Mr. diGenova at least has not taken that position.

There's nothing wrong with the 500-foot rule at embassies as a uniform protection. But the law should be enforced without the arbitrariness that is apparent here.