Because the turrets designed as a deterrent, rather than a weapon?
It's like a mock security camera. People don't commit crimes when they think they're being watched, even if they aren't. Likewise, if an automated machine gun let's people know where it is and has a big ol' laser pointer telling you where it's looking, it doesn't need to shoot to scare off most people.
Think about it, how many of the turrets in Portal 2 could have been incapable of firing, and you still would have avoided them?
(Aside from the Naff turrets, of course)
It's like a mock security camera. People don't commit crimes when they think they're being watched, even if they aren't. Likewise, if an automated machine gun let's people know where it is and has a big ol' laser pointer telling you where it's looking, it doesn't need to shoot to scare off most people.
Think about it, how many of the turrets in Portal 2 could have been incapable of firing, and you still would have avoided them?