I think because they have the basic (human) desire to live, and their basic (human) rationality sees that medicines and science will help them, but at the same time their nurture (what they've been brought up with), their gained beliefs, mixed with the fact that they don't understand how the medicine they're taking works, are conflicting with this.
As Richard Dawkins says (something like) "you'll find that people say they're religious in a church, but as soon as they're in a hospital they become faithful in their doctors and their sciences"
The ones who speak against science negatively are probably very insecure about their beliefs. Despite everything science has done for us, to then go against it whilst still trusting it to save you shows real insecurity in your other beliefs.
Also on a slightly related note, I've heard, and no doubt a lot of the responses to this will be something like "God created science and doctors and so science doesn't conflict with God."
Though I think they're wrong when they decide to still be Christian, Muslim or Jewish, because science greatly challenges their texts. Perhaps if God was limited to creating the Universe and that's it, maybe that argument could be made, but a personal God is surely not possible.