I don't see what the big deal is myself. I mean considering blood always has to be verified by two nurses, anyway. I'm not a Jehovah's Witness. I'm an agnostic, so I have no dog in this fight, but come on!
Now, dear OP, I understand your religion is important to you, but you don't have to discuss it during an interview. Next time, keep it to yourself. You can't be discriminated against because of your religion. Plain and simple. But, you must also realize what nurses do in every unit. We take blood, we give blood. I'm in Med/Surg and we give blood all the time. What's in your favor is, there are always two nurses to verify the blood. If the only thing you have a problem with is pushing the start button, like I said, what's the big deal? Geez. I'd be happy to push the start button for a Jehovah's Witness. I'd be happy to connect the line. If there is such an emergency that the blood has to be administered within seconds, which is impossible considering the logistics one must go through to get blood from the lab, etc., I can't imagine there wouldn't be more than one person in the room at the time, anyway.
Now, if you were trying to get a job in an abortion clinic and you're against abortion, that would be a different story. Or, if you don't believe in birth control and you apply for a job at Planned Parenthood, I'd say that was a problem. I just don't see the problem here. If I were you, I might talk to someone who knows something about the law. You might also want to look inside yourself and see if you could be as open-minded about this situation as you're expecting everyone else to be. It's a two-way street. If someone's life and health were on the line, could you push the damned button?