I've read that the reason America's mega-fauna went extinct was over hunting because they were easy prey because they hadn't evolved alongside humans and so didn't know to fear them (or some such). I figure that if this is true, we should find scads of butchered carcasses of American mega fauna lying around, but AFAIK we haven't. Is this theory bunk?
This really is more of an ecology question, but I studied ecology, not history so here we go.
The short answer is that in the case of north and south American (and Australia) mega-fauna humans were most likely the primary cause of widespread extinction. Quite a lot of work has gone in to trying to suss out the role of environmental change in the late quarternary extinctions and currently the best answer is a resounding shrug. Climactic change seems to have played an outsized role in the extinctions of some of the Eurasian mega-fauna, but on the whole it isn't an adequate explanation for why these extinctions were so skewed towards large animals. The fossil record doesn't similar mass extinctions of large animals in previous interglacial periods, and island ecosystems like New Zealand seem to have had no problems with their large avians until the much later arrival of humans.
So we have to talk about humans. First and foremost we should probably dispense with the idea of prey being, and remaining naive to human predation, because it's almost certainly not the case. However, what may be true for some genera of large herbivores is a prominent evolved "fight" response, which would be maladaptive when faced with humans with pointy sticks (giant ground sloths, for instance, likely tried to defended themselves with their gigantic claws rather than run ... they weren't, you know, very fast). Most of these animals were subject to predation, if only as juveniles, what with all of the saber toothed cats and freakin' Dire Wolves running around, so the idea of them passively being hunted to death by humans is fairly ridiculous.
However, what we can say about the majority of the mega-fauna that we're interested in here, is that they all had slow reproductive cycles, and therefore slow replacement rates. The Blitzkrieg theory (yes, they actually call it that) of humans arriving in the Americas and Australia and wiping out the mega fauna in a couple of generations is most likely incorrect, but a more nuanced approach of whittling mega-fauna populations down over the course of a couple thousand years (I never clairfied this point - we're talking between 50,000-10,000 years ago) seems more plausible. The crux here is that humans are opportunistic predators - when large game is available to hunt we will hunt it vigorously, and when it isn't we can scrape by on small game and foraging, thus avoiding a boom and bust prey-predator cycle that one would usually see in most ecosystems. When coupled with climactic and environmental upheaval this is currently our best explanation for the Late Quarternary Extinctions.
You may want to ask this in /r/askanthropology
The argument above is presented as though it was settled science. It surely is not. For an example, see this article by Don Grayson and Dave Meltzer that argues very persuasively that there is simply no evidence to support the notion that paleo-Americans were responsible for quaternary extinctions. Even the Wikipedia page on Quaternary extinctions is more balanced than this. Take a look at /r/AskAnthropology right here and here. Also, I know of no published article that suggests, authoritatively, that paleo-Americans were in the new world 50,000 years ago. So if the extinctions started 50,000 years ago, and Paleoiandians got here 20-15,000 years ago, what accounts for the early extinctions?