Citizenaid is a combination of common sense. Learning a few simple procedures and a few first aid techniques to save life. Probably the biggest difference between your standard first aid course and citizenaid is about improvisation.
Unfortunately, the aftermath of a terrorist bombing or a shooting attack is grim. A member of the public will be confronted with lots of casualties all in 1 place with a range of life changing injuries. Psychologically it is difficult to deal with this without some form of training. Why do the armed forces train with amputees? Because its realistic training and it conditions the mind to deal with the real thing.
Terrorists attack us when we are the most vulnerable in a crowded place? Because the chances of killing and maiming is easier and the attack will create greater fear.