Go ahead and throw your grandma the responsible keeper response. The only reason I could ever possibly think of for a 12-20 ft snake to go ahead and try to eat it's keeper (an average size adult human -- too big for a snake that size) would be because it's not being fed as often as it should, and so when the owner opens the cage and smells like a feeder... Also, throw in some statistics. Owners or family members being killed by their snake is something that doesn't happen everyday. For lack of better comparison, it's like being attacked by a shark (which happens more often than snake attacks do anyways), hardly ever happens but when it does everyone's suddenly terrified of swimming and biased against the animal. Or you can use the "you've got a better chance of being mauled by your neighbors dog than being attacked by a python" response. If you're a responsible owner who knows what you're doing, you've got a better chance of winning a powerball prize than being squeezed to death by your pet python.
I've been asked the same question so many times (or some variant of it) that most people have to ask it twice in order for my brain to even register it anymore. I do what I can to dissuade fears and educate the people who take the time to listen (I can't even begin to count the amount of times I've had to tell people ball pythons don't get gigantic). When I got my first ball python, all of my friends bugged because they were afraid, and now half of them are tolerant and the other half either own their own snakes or want to own one.
Just throw the facts out there. Educate who you can, and they'll either come around or they won't.