But in genuine "hard science," scientists have their preconcieved notions overturned all the time. That's the whole purpose of scientific protocols -- to remove personal bias from the equation and measure things by a higher and more objective standard.
Even without the protocols, good scientists should be able to leave preconceptions at the door, like good judges in the courtroom. It's really not that hard for a genuine professional to recognize that he has a personal bias and determine whether he can leave it outside the laboratory or courtroom for work purposes or whether he should recuse himself if he can't.
On a related note: I'm not an atheist because I like being an atheist or because I somehow became "entrenched." It's just that I wasn't raised in a faith as a child, so as an adult I'm going to need something pretty damned solid before I can buy into that particular philosophical nightmare.