How is this a "well established principle of law"?
I would have to check the US authorities but the Supreme Court of Canada has stated this on many occasions, the leading case is The Queen v. Morin [1988] 2 SCR 345. The court stated the point this way (p. 362):
"The jury should be told that the facts are not to be examined separately and in isolation with reference to the criminal standard. This instruction is a necessary corollary to the basic rule referred to above. Without it there is some danger that a jury might conclude that the requirement that each issue or element of the offence be proved beyond a reasonable doubt demands that individual items of evidence be so proved."
If each piece of evidence individually doesn't meet a reasonable doubt standard, then how could they possibly meet it when combined?
It is very simple, common sense. One can reach a conclusion beyond a reasonable doubt about a fact based on many independent pieces of evidence that point to guilt but do not individually prove guilt.
For example, suppose the issue is identity of the killer and 16 witnesses independently describe 8 different things about the identity of the person who committed the crime (he wore a baseball cap, he had a beard, he had a bleeding cut on his left hand, he spoke with a french accent, he had blonde medium length hair, he wore blue running shoes, he had a blue denim jacket and he drove away in a red pickup truck with a damaged right tail-light). None of the witnesses were 100% sure that they made correct observations. Now it so happens that a man fitting that description was stopped about a mile from the scene of the crime within a two minutes of the crime being committed, driving a red pickup truck with a damaged right tail-light. It also turns out that the accused had been captured on video in a bar drinking with the deceased earlier in the evening.
Each one of those pieces of evidence cannot be proven beyond a reasonable doubt by itself nor can a single piece of evidence prove the identity of the accused as the killer beyond a reasonable doubt. But together they form the basis on which a jury could conclude, beyond a reasonable doubt, that the accused was the killer.