Maybe I'm just being obtuse but why would the Arabs object to the installation of metal detectors which could only make their sacred site safer for everyone - Muslims even more than Jews?  Are they saying we would rather be victims of a crazed gunman than protected by Jews?  

Obviously the Israelis are primarily concerned that weapons smuggled into the Mosque would likely be used against them  but given the degree of internecine violence among Muslims throughout the Islamic world it would certainly protect the worshipers in the mosque from any potential violence.