The Leftwing attack on the Midddle East Research Institute (MEMRI)

I wonder why does every crack-brained accusation about Israel get a serious hearing while explanations of the facts get dismissed out of hand as Zionist propaganda? Why did a British newspaper editor write that he no longer pays attention to pro-Israel information if the letter writer's name sounds Jewish? Why do people consider it an adequate refutation of MEMRI's (Middle East Research Institute) work to "accuse" MEMRI of being founded and run by Israelis Zionists (which is true) and of running inaccurate translations of the Arab press (which is not true), without pointing to a single inaccuracy in any of MEMRI's translations?