Explanation. Your post seemed to wonder why fear-based arguments (which, again, I want to state are not my arguments) would be put forward, and I’m saying that history has shown people that fear-based arguments are often successful in winning mass support, and that (all other things being equal) forces will tend to follow the path of least resistance. That you’re offering resistance to the fear-based method is a good thing, because it makes it less likely that people will follow that path in the future, but I read at least a portion of your post as asking why anyone would think that such an argument would work, and my answer is that the argument has nothing to do with whether or not it is correct, and everything to do with whether or not it has historically secured the result that the person arguing it desires. If I misread the intent of your post, if you did not in fact wonder why anyone would use such an argument, then I withdraw my comment.

Explanation or excuse?  Your exposition seems, essentially, to be “it’s really tough for these people”.

I don’t care.  A lot of things are tough.

So, it might be hard for religionists to believe otherwise.  They may want to cloister themselves and repeat over and over that they are the thin pearly line between society and the rise of Satan or whatever other mythical boogeyman they, as adults, are afraid lies under the bed.  They may want to set up their own schools and refuse to teach science or anthropology or history because it contradicts an antique book or their own myopia.  Great.  Have at.  But there is one thing you cannot expect from me when I exercise tolerance in this realm, and that is respect.

In these situations, I am often struck with the idea that I must be completely missing some subtlety

To be clear, your point is that I am not missing any subtleties, right?  This does not mean that no such subtleties exist, of course, but, to be clear, you are saying you do not believe there are any?

The notion that “without x we would all be monsters”, where x has a value equal to whatever the speaker holds dear, is neither new nor particularly confined to religion (although the religious community has always recognized its utility as an argument). How many times have anarchists been told that without a system of hierarchy codified by laws and maintained by force, we would immediately descend into a non-stop orgy of rape and murder? What was Children of Men, if not a warning that we would revert to either barbarism or tyranny if we didn’t maintain our drive to create smaller versions of ourselves? Haven’t transhumanists been told time and again that in attempting to surpass human limitations they are threatening to become something worse than human (as though such a thing could exist)? Isn’t the drug war predicated on the idea that we’d all be selling our daughters for crack if it weren’t for the billions that we pour into their budget?

I’m of the opinion that all ethical systems are essentially survival strategies for societies. Sometimes they work very well (“Share your crops with your neighbors”). Sometimes they need to be revised or dropped as society evolves (“This class of people can be the property of this other class of people”). Sometimes they backfire disastrously (“This group of people is more deserving; let’s invade Poland”). The thing to keep in mind when dealing with arguments like the one we’re discussing is that when people are being asked to revise the ethical system that they know, they’re being asked to drop their society’s current survival strategy. Given the stakes, that’s expecting quite a bit of them. A lot of societies have had religion as a part of their strategy for a very long time, and it has worked (or at least been non-disastrous) in the case of the surviving ones. So for a lot of people, it’s impossible to envision society turning away from the ethical framework that they’ve always known without it having horrific consequences.