THE Western hostages in Lebanon have been imprisoned as much by the desperate politics of that country as by their terrorist abductors. Now that political scene is changing.The families and friends of John McCarthy and Edward Austin Tracy have had their hopes fulfilled, and further releases may follow. Much depends on United Nations mediation between radical groups in Lebanon and Israeli authorities, who hold Arab prisoners. Will Islamic Jihad and others give an accounting of Israel's servicemen still missing in Lebanon? What does their demand for the release of all the world's detainees really mean? Lebanon's pacification in recent months has helped open prison doors for the hostages. The Syrian-backed government has pushed its authority into areas previously controlled by guerrillas. The diligence of Lebanese police in combing Beirut for Frenchman Jme Leyraud, seized last Thursday, hastened his release. Shortly before the release of Mr. Leyraud and Mr. Tracy on Sunday, a prominent Shiite leader, Sheikh Mohammed Hussein Fadlallah, renewed his call to free the hostages. Syria and Iran, who in the past supported the hostage-takers, are trying to overhaul their policies - and their images - in an attempt to gain Western favor. Syria has hopped aboard the US-led effort to organize a regional peace conference this fall. Iran, under President Hashemi Rafsanjani, is veering from the isolationist course preferred by the hard-line followers of Ayatollah Khomeini. The role of the UN is central. Iran and its Shiite clients in Lebanon want the political insulation of negotiating with the UN, rather than with the West or Israel directly. Incentives to resolve the hostage issue have never been stronger. All sides feel pressure to contribute to a solution, particularly with a broader Middle East peace initiative gaining momentum. Sheik Fadlallah's remarks emphasized that the "negative consequences" of hostage-taking now outweigh the "positive ones." In fact, there never were any "positive ones." The seizing of hostages brings only condemnation to the abductors and their causes. This hostage crisis should be the last.