Most JWs do not know about this. Just recently I came across one who didn't know they can get fractions from blood or that JWs are allowed to take them.
They trust a group of men to tell them what they can and cannot have without giving any thought or research into it. They trust that the research has been done for them and that the organization would not allow anything that Jehovah disapproves of.
Logic does not come into it.