Though I will ask that you do not make the dangerous assumption that "unarmed = not dangerous" or that being found to be unarmed post-facto made them not dangerous to begin with.
If someone threatens to have a gun and threatens to shoot but is actually unarmed at a distance of 30' and they reach behind them - what do you do? Waiting for visual confirmation of a weapon could leave you dead. The potential of them not bluffing is there.
That scenario is a little contrived but there are many variations and they don't involve the suspect having a weapon or bluffing to have a weapon. What if they don't say anything at all but reach behind them? Does that change the situation?
>And it also seems odd to me that anyone would be surprised that officers might be injured
I'm not surprised at all, it is expected to have risk after all. I am surprised that people are asking officers to take additional and unnecessary risks.
1: http://www.odmp.org/search/year 2: http://killedbypolice.net/