BY WHAT SORT of self-indulgent pseudo-idealism did the State Department decide to rescind its own decision of March 15 to let tear gas be sold to Pakistan? "Conditions have changed substantially in the four weeks since the license was issued," the department says, by which it means that protests against Prime Minister Bhutto, whom the opposition accuses of having rigged the elections of March 7, roll on. It is apparently the departments's view that further deliveries of tear gas could be interpreted as endorsement of the Bhutto government at a moment when its end may be near.

This is absurd. Pakistan has been a good friend and longtime treaty ally of the United States for decades. Mr. Bhutto, who took over after the East Pakistan was torn away in 1970, has since been credited with genuine achievements in settling his country down, pushing development forward and, yes, enhancing human rights. He does not take orders from Washington, and he has made mistakes at home, but he is unquestionably one of the best Third World leaders currently going. It is his government that Washington has stopped providing with a normal and, under the circumstances, humane means of coping with street disorders; one alternative, of course, is bullets. The political insult is plain.

Aha, say the pure of heart, he rigged the elections. Did he? Has the State Department so established? On March 15, after all, one week after the elections, the department was prepared to ship the tear gas. Whatever Mr.Bhutto did, was it more heinous than what went on in, say, Cook County in 1960? It strikes us as both arrogant and pusillanimous of the department to have decided that Mr. Bhuttto's electoral performance, whatever it was, negated the many other ties between the United States and Pakistan and required the Carter administration to deal him, at a moment of his extreme duress, a small but savage vote of no-confidence.

We are not unmindful of the new administration's effort to take a fresh look at arms transfers. Some foreign governments are bound to get caught in the gears of American policy change. We offer no endorsement of blind acceptance of past arms-transfer patterns. But in this case a major error has been made. Apparently intent on not appearing to be interfering by sending a pro-Bhutto signal to Pakistan, the State Department seems not to see that it is interfering much more blatantly by sending an anti-Bhutto signal. That is the way Pakistans will surely read the department's conspicuous departure from the normal course: a routine approval of the tear gas shipments. And the department seems equally blind to the fact that it is also sending a lot of other countries the wrong message - one that says that when otherwise unoffending friends get in trouble, the Carter administration quickly gives them the back of its hand. The tear gas shipments should be promptly resumed.