Indeed! You have successfully grasped the concept, well done. A brief peruse of
Wikipedia shows that not only is 'innocent until proven guilty' framed and based in the reasoning I said it was, but that it also stems directly from the concept of the onus of proof.
No, you
have to believe in the dinosaur and the unicorn. It's not simply a case of, "Oh, maybe they exist, maybe they don't" -according to your logic, we must logically assert the existence of the dinosaur and the unicorn, as real and as firmly as anything else, simply because no one's disproven them.
Obviously, they
may exist. But simply asserting that they exist is not enough to justify belief in them - you have to have evidence that they do, in fact, exist. Otherwise, the null hypothesis - they don't exist - is what we logically lean towards. Non-existence is the preferred choice due to Occam's Razor, by the way.
And, as it happens, there's good evidence that the dinosaur and the unicorn don't exist, as absence of evidence
is evidence of absence: if a hulking great dinosaur exists, I'd expect to see poop. If I see no poop, despite looking high and low, I'm justified in rejecting the claim that they don't exist.
This is demonstrably fallicious reasoning. Consider the following negative statement:
"There are no married bachelors".
This is a true statement, and it is known. We can logically and absolutely prove that there married bachelors are non-existent. Moreover, in the realm of science, we can demonstrate to exceedingly high degrees of accuracy that something doesn't exist - the luminiferous aether, a living
T. rex, etc. Your logician's approach only means that we can't disprove most things with 100% certainty - but neither can we
prove them. Ultimately, we need to have standards of reasonable proof, and with those standards we
can prove and disprove something.
Atoms, for instance. There's almost the minutest chance that it's all magic gnomes or aliens or God, but that's unlikely. We have proven, beyond
reasonable doubt (but not
all doubt), that atoms exist.
So, with relation to negative argumentation, your logician's approach is simply repeating the obvious. We
can justify a belief that something doesn't exist, and we disprove it with 99.999999...% certainty, we just can't always mathematically disprove it to
100% accuracy. Which is fine. But don't think that we can't do anything to knock or support a negative claim.
Short answer: your logic is flawed (you must, by your logic, affirm the existence of unicorns in my garden. I trust that you don't).
Which is why we atheists chuckle when a theists accuses us of just being mad at God - as you astutely pointed out, atheists don't believe God exists, so how could we be mad at him? Are you, a Christian, mad at Vishnu? No: you simply don't believe he (she?) exists.