Because, if I am reading the article correctly, some hydrogen (which has become radiation) will go through you anyway. This kills you. Somehow gathering up all of the radiation and trying to use it to fuel a ramjet is the same thing as trying to make some sort of shield, which the article says would be very difficult.
I think the idea is that it doesn't ever touch your ship--you just funnel it into your reactor directly. (You have to ionize it somehow...) So it's not equivalent to shielding--it's designed to avoid shielding.
> So it's not equivalent to shielding--it's designed to avoid shielding.
Not quite. Electromagnetic shielding is a potentially viable thing, and can be much less massive than material shielding; a Bussard Ramjet doesn't avoid shielding so much as repurposes the electromagnetic shielding you already needed anyway.
Ionizing is not that big of a problem. If you're going fast enough, hydrogen atoms will be ionized by the interaction with your electromagnetic field. If you're not going fast enough, you can shine a maser ahead of you to ionize things in your path (which cuts in to the efficiency of the ramjet).
