Question of fault aside, I’m not sure you can say the policeman could know the bikes could stop in time as that clearly wasn’t the case.
What would have happened if the first rider had panicked and decked it or slipped on an unnoticed hazard and hit him? He has a duty if care for his own safety and members of the public, I would also say he failed spectacularly.
From a risk management perspective that was utter stupidity on the part of the policeman, if you did something similar in an industrial environment you would get a serious bollocking at best or put off site and I’ve seen both of those happen for far less.