Question

3. Why would someone who rides in an ambulance carrying a patient with an airborne communicable disease be considered to have been exposed?

Answer 1

Because he obviously got the airborne disease from someone else and is therefore got it from someone else and is considered "exposed" and should be treated so 

Answer 2

If the disease is communicable, it is contagious. If it is airborne, it is transferred through the air, as opposed to physical touch or blood-borne. Being in a confined space, such as an ambulance, with an airborne communicable disease means you have most definitely been exposed to it.