Errrm, they can.
My knowledge of the legal side of this comes from hitting a "fellow" cyclist. On a bike if you haven't got time to stop, the law will hold you as travelling too fast for the situation you're in. It's harsh and I have a lot of sympathy for Lilac and personally I would probably have been doing exactly the same thing. However the law will hold it that on approach to a pedestrian crossing regardless of its current state, you should be able to stop if a ped steps out.
I think we are talking about what is a reasonable circumstances. Or put another way, is it "foreseeable" that there may be an accident if you carry on as you are? If you are rollling along at 16mph and you see a pedestrian standing in the road up ahead, then you must stop because it is foreseeable (or you know) that you will hit them if you carry on. It doesn't matter what colour the lights are etc etc.
At the other end of the spectrum, if you are rolling along at 16mph it is not foreseeable (you don't know) that you are about to hit a pedestrian who jumps into the road from behind a hedge 2 feet in front of your bike. I think this follows what gambatte is saying.
I think Lila circumstances lie somewhere towards my second example. As Lila approached the lights, the ped was on the pavement and the lights were on green and the red man was on. It was not foreseeable that the ped would step into the road so Lila does not have to slow down. If we had to slow down for every ped just in case they step into the road, we would all have to drive around in 5mph marshmallow cars.
The same goes for cars - If there are people stood either side of a crossing and the lights are on green, a car does not have to slow down to 5mph. If a ped decides to step into the road and it is too late for the car to stop then it is not the car drivers fault. (Sorry Martin235, I think this is where I disagree with you). Alternatively, if the peds are already starting to cross as the car approaches from 200yds away then the car must slow down regardless of the colour of the lights.
Likewise, if you are drafting the bike in front and they hit the brakes and you go into the back of them, it was foreseeable when you were drafting that you would probably hit the bike in front if they had to slam on. The same applies to cars - If you tailgate another car and they have to slam on and you go into the back of them, it is your fault.