Quantitative determination of risk is a very complicated science; in fact, it's more of a 'black art'. However, in general terms, "Risk" (R) is the product of "Potential Loss" (L) and "Probability of Occurrence" (P):
R = L × P
Now, in the case of PED interference causing a plane crash, I would be the first to agree that "P" is very small; however, "L" is huge (potentially, 400+ fatalities on a reasonably large aircraft). So, although it is very difficult to quantify "P" (even relative to other risks), the airline industry has to respect the formula, and err on the side of caution.
-- Henry