A large part of the problem is that most people aren't aware of the broader philosophical context.

While atheists don't believe in god, whereas theists do, most people in both cases aren't aware of metaphysics and the arguments for and against the various metaphysical theories. As a general rule, most atheists are Materialists but don't fully understand what that means and aren't typically aware of the counterarguments against that metaphysical theory.

The biggest part about this that bothers me, personally, isn't so much that they think their beliefs are epistomological, but rather that they believe they have no metaphysical worldview (largely because they tend to have hangups about the term "metaphysical" without actually understanding what it is). The fact is, everyone has a metaphysical worldview, whether they realize it or not, or understand it or not, and that metaphysical worldview directly informs their other beliefs, including epistemological ones. But atheists tend to have a certain degree of snobbery (sometimes even to the point of elitism) regarding their own metaphysical worldview (Materialism) without even understanding (and often without even being aware of) the arguments for and against that philosophy.

I don't think I would go so far as to say that is a straw man, so much as just philosophical ignorance.

Essentially, they are working backwards (from a philosophical point of view). They accept Materialism without understanding it (and often without being aware of exactly what Materialism is and means and what its implications are), and therefore accept evidentiary skepticism as the most effective methodology. Then they assume that if you cannot show evidence, then it must not be, despite that being logically invalid, instead of the logically valid position of "there is no evidence, therefore it might not be (and perhaps even probably isn't), but the possibility still remains, and I choose to only actively believe those things that there is evidence for." Then they make their combined ontological/epistemological claim: "I do not believe in god because I see no evidence for the existence of god." But all of this is predicated on an unproven (and as with all metaphysics, unprovable) metaphysical theory of Materialism without understanding the metaphysics.

This doesn't really set them apart from theists in terms of philosophical ignorance, though, because theists reject Materialism without understanding it and without knowing or understanding the other metaphysical theories (and their problems) either. They simply accept that there is a spiritual/supernatural aspect to the nature of reality and then allow that belief to inform their other beliefs as well.

Now, if an atheist made an actual argument...

If Materialism is true, then evidentiary skepticism is the best methodology. If evidentiary skepticism is the best methodology, then only that which has evidence should be accepted as fact. Materialism is true, therefore evidentiary skepticism is the best methodology, and therefore only that which has evidence should be accepted as fact. If there is no evidence for the existence of god, then the existence of god should not be accepted as fact. There is no evidence for the existence of god, therefore god does not exist.

That is: If A, then B. If B, then C. If C, then D. A, therefore B (despite the fact that A is unproven), therefore C, therefore X (a conclusion that isn't supported by the premises). Thus, they would have essentially committed the fallacy of presupposition (a form of "begging the question" without requiring it to be in the form of a question), and the hasty generalization/jumping to conclusions fallacy.

If someone actually presented such an argument (albeit almost certainly not that formulaic), then it could be said that they had presented a fallacious argument. In the first place, their acceptance of the premise "Materialism is true" hasn't been fully supported and can be questioned, so all of their conclusions become automatically dubious without additional support for that base premise. And then in the second place, the final conclusion goes too far because the premises do not require that specific conclusion (even if those premises are true).

But generally, atheists tend not to make that specific argument. They accept those premises and conclusions, but without making the actual arguments themselves.

Personally, I just wish more people would learn to understand metaphysics and ontology, particularly as they inform all other aspects of philosophy, including both spiritual philosophy (or anti-spiritual philosophy) and Ethics. But seeing as it is one of the more difficult branches of philosophy, I can't say I'm surprised that so few people do.

 "To claim that atheism is epistemology but theism is ontology is a straw-man fallacy, *since epistemological claims are more easily defeated than ontological ones.*"

I wouldn't say it's a straw man because "epistemological claims are more easily defeated than ontological ones." I'd say it's a straw man because people are misrepresenting atheism.

I'm not even sure I'd say it's a straw man period. No argument is really being deliberately misrepresented or misinterpreted. People genuinely believe it to be the case. And there are even arguments to support it. The problem is within semantics, not argument.

Do I understand you correctly? Atheism supposedly makes no claims nor holds beliefs. Yet there are certain assumptions about reality that are generally associated with atheism—that humans are capable of discovering the nature of reality, that reality can be understood by reducing it into parts, that there is an objective, material reality consisting of actual “things” that we perceive with our senses, and that sensory input can be trusted to reveal the nature of that reality.

Maybe not all atheists make those assumptions. I suspect that those assumptions are made by many who say they believe in God. I wonder if by getting rid of those assumptions, the atheist/theist arguments would cease abruptly while people simply sat dumbfounded, realizing that they know nothing except superficially.