Some of the sensors sitting on top of lights aren't there to catch people jumping them. Believe it or not, they're for emergency service vehicles and they sense the approaching flashing lights (headlights) and this alters the timing of the lights which change in favour of the emergency vehicles' right of way.
To test this, the next time you come up to a set of single A/B road temporary road works traffic lights, flash the lights intermittently as you approach (if you're first in teh queue). After a safe time to allow oncoming traffic to clear, the lights will change for you, thanks to the lil light sensor box sitting on the top.
The other part of the reason for this is that the emergency services aren't imune from prosecution for a driving offence if they jump a red even under blue light conditions. It builds in an element of safety for them.
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