Same way they do, you ask for the full tape, which you can get. Or you know just go around the wrong way. Anecdotally red light covered cameras seem to be much better maintained as othewise they lose revenue.
The red light cameras I've seen trigger when someone approaches a red light without slowing down quickly enough. Sometimes, late at night, I make a game of setting them off on purpose, by waiting til the last minute to brake. They wouldn't fire if you stopped, waited, then continued after determining that the detector hadn't noticed you.
Also this is why you have a dash cam.