That's just a fundamental axiom of logic, one whose practical effects you learn very quickly when you hear PROSECUTORS saying, "an eye witness is the worst witness there is" and DNA evidence that only matches 1 in 20 billion people. You don't even have to get into the Critique of Pure Reason or Heisenberg, though you certainly COULD if the standard were "100% certainty, 0% doubt", and no one could ever be convicted of anything. If you'll notice, even in this case the suspect is still being referred to as just that, and his role in the attacks as "alleged". Yes, that's to cover the medias butt against libel charges but consider: If he's found guilty of murder they can't be sued for calling him a murderer; they have to say things like "alleged" because it's entirely possible, however unlikely, that he won't be convicted.

If you want further elaboration on the meaning and justification of the term "reasonable doubt" ask Tom, but if your grasp of its meaning and basis in law is genuinely that poor you're in no position to say what should and shouldn't merit capital punishment.