CIA uses U.S passports, they don't need to use fake foreign passports. So do most countries. The mossad uses different passport because the Israeli passport is not accepted everywhere, especially Muslim countries. An Israeli citizen going to a Muslim country?! Why?! Mossad who work in USA, use Israeli passports to go in. Another reason is, Israel treats itself above all countries when it comes to international law, it is the spoiled child. However, other countries care more about international law.
Kenya police: 2 Kenyans arrested for spying for Iran (AP)
from the article: Britain 'knew Mossad was using fake passports for Dubai hit'