If you want to treat this as an engineering tradeoff, then you have to not only measure deaths but property damage.
Myself I've never reversed into a human being, but I have reversed the car into 1) dozens of other bumpers in tight parallel-parking spots, 2) a fence 3) several curbs 4) the side of a car, 5) a stone wall, and just two months ago 6) a trailer hitch. All those dings and dents cost money, and are much easier to assess than the actuarial dollar-value of 15 deaths.
The real scandal in this news, though, is that the NHTSA has delayed crafting this simple rule for so long. The law was passed in 2008 with a deadline of early 2011. The Obama administration delayed the rulemaking for so long presumably because most auto makers make money selling cameras as optional equipment. The NHTSA gave the excuse that they needed time to do a 'required cost-benefit analysis' of the 15/deaths per year against the $150 cost of the camera. What the heck takes so long? Congress already passed the law requiring the cameras. All NHTSA had to do is take out a piece of letterhead, write down "10 million cars/year * $150/car / 15 lives/year equals $100 million/life", sign it, and file it away.