AF illuminator on A55, is it normal?

The AF illuminator on my A55 works only when it is in Auto mode. It doesn't fire in any of A,S & M mode. Is this normal for A55? Ok, one can understand that in Manual Focus it may not fire but I don't understand why it should not in A,S & M modes? Why have they kept it like this. So how do you AF when you are in A,S or M in a very low light situation? I have not used any other SLR. What about the cameras which have dedicated AF lamp like A700 etc.? Do they fire always irrespective of which exposure mode the camera is in? Please clarify me. Thank you.