Because Al-Qaida terrorists may pose as American Jews to sneak into Israel. Or may pose as [your nationality here] Jews. As long as The Enemy is invisible, like for instance invisible WMD's, it is of immense use in either global warfare, or local policies, to gain support unpopular policies.
Law expanding definition of terrorist activity passes first Knesset reading (Haaretz)
from the article: 'Al-Qaida terrorists may pose as Ethiopians to sneak into Israel'