NOT LIVING under a parliamentary system, we perhaps do not fully understand all the niceties that Prime Minister Menachem Begin evokes by way of explaining why it would be distasteful and even impure for him to try to influence his parliament's forthcoming vote on dismantling Israeli settlements in the Sinai. The Knesset's decision two weeks hence, he holds, will be "completely free," taken with "no party discipline," done by each member "in accordance with his own conscience." He presents this view as a sign of reverence for a venerable political tradition, something for which he should be indulged.

His personal detachment, however, is something else again. Mr. Begin himself concedes that his own foreign minister finds his diffidence "a little bit curious." It is more. It is, in a word, pusillanimous. We are not sure whether it is Mr. Begin's Zionist commitment to pioneering and settling, or an obligation to certain constituencies, or something else that accounts for his refusal to speak out for removal of the Sinai settlements. No matter. He has a responsibility to work for his own government's programs, of which none can now have a higher priority than the successful conclusion of an Egyptian-Israeli peace treaty. To say, as he does, that "98 percent" of the treaty has been negotiated is to ignore that the completing "2 per cent," without which the rest is meaningless, lies exclusively in Israel's hands.

Without removal of the settlements, Egypt would have reason to say that Israel had broken faith with its Camp David undertakings. That would be the end of the whole process set in train at the summit. "This settlement issue," Anwar Sadat says, "is a matter of principle for me. It is not something we can compromise on . . . . This is something that I can't agree to, or afford." We entirely agree.

But you may say, surely the Knesset, even voting "freely," will play out this charade in a manner befitting the stakes in the Middle East. Probably so. But that does not still all our misgivings. It is troubling that Mr. Begin would be ready to cast the impression that, rather than sully his own personal record, he would buck to his parliament the admittedly difficult but essential business of closing out the Sinai settlements. That is a cold response to the generous and fundamental adjustments Mr. Sadat has made in positions he had previously taken - as a matter of principle. Mr. begin might better have welcomed, or at least accepted, the responsibility to show leadership and give inspiration to Israelis - and to Egyptians, too. The outcome of the Knesset's vote may not be in doubt. But there remains the crucial matter of the climate in which Israel's subsequent negotiations, with Egypt and with Jordan and the Palestinians, unfold.

Mr. Begin's approach to the Knesset vote is out of character with the statesmanship he showed at Camp David. We hope he will reconsider it.