Because they were muslims and are allowed to lie to present the evil religion in good light to gain converts by deceiving. Any non-muslim who has studied the religion knows without doubt that it is anything but peaceful.
Actually, they're allowed to decieve to defeat their infidel enemies, because infidels follow Satan, and Satan lies to the infidels, so they're supposed to do what Satan does. Make nice promises, give presents...then stab and kill when the enemy is lured in. Sound familiar?