Principle number 22 of Basic Elements of Islam: The Quran counsels the use of deceit when dealing with non-Muslims. Mohammad instructed one of his followers to lie if he had to (in order to assassinate one of Mohammad's enemies). This set a precedent, and the principle was clear: If it helps Islam, it's okay to deceive non-Muslims. Read more about this principle here: Lying (Taqiyya and Kitman). And here.
This instruction in the Quran has served Islamic goals very well through history. And it serves those goals today. On the DVD, Obsession: Radical Islam's War Against the West, you can watch real-life examples of Islamic leaders saying one thing in English for the Western press, and saying something entirely different to their own followers in Arabic a few days later.
Deceiving the enemy is always useful in war, and throughout history generals have used it. Islamic teachings consider Islam to be in a permanent state of war with the non-Islamic world until the whole world follows Sharia law (read more about that here). All non-Muslims living in non-Islamic states are "enemies." So deceiving Westerners is totally acceptable because deceiving an enemy in a state of war is totally acceptable. It is encouraged if it can forward the goals of the spread of Islam.
And so we have the strange phenomenon covered by Steven Emerson in Terrorists Among Us, where organizations in America were ostensibly raising money for orphans, but really giving the money to terrorists. They deceived good-hearted Western non-Muslims into giving money to organizations that were actively killing Western non-Muslims.
As it says in the Quran, "War is deceit." This idea gives Islam a tremendous advantage over idea-collections that encourage indiscriminate truthfulness.