My mother and I spoke about this the other night after it came on the news. We both came to a conclussion about some of these instances: We don't think people expect DISNEY thrill rides to be as "thrilling" as other parks. You go to Universal and you expect it at their parks, so I think some people take more precautions and read the warnings a bit more seriously. "RnR can't be as bad as the rides at Universal because this is Disney!" I have actually heard people say this when I talk to them about rides like RnR and M:S.
Disney does all it can to keep it's guests safe. They post warnings and impliment height limits for many of their rides. However, they can not force people to read those warnings and take them to heart. If someone refuses to heed the warnings posted, for whatever reason, then what can disney do? You can not look at someone and say "oh, they have high blood pressure so they can't ride." when they get to the loading area. You have to trust they have made a correct decision when they decided to ride.
An undiagnosed health issue can hit anyone, anywhere. Tomorrow I could have a blood clot hit my brain... but I am not going to let that stop me from living my life. I refuse to allow the "what if"'s control me and my decisions. It is up to the parents best judgement on if their child should ride something. My mother told me she would not have let one death in millions and millions and millions keep me from riding if I had wanted to if I was still that young. If she did that, then she would have had to never let me ride in a car or on a plane.