(1) the text on that sign in Illinois?
For convenience, there's the text:
"At the time of the winter solstice, let reason prevail.
While I'm all in favor of reason, I see no need to highlight the fact that it's the winter solstice.
There are no gods, no devils, no angels, no heaven or hell. There is only our natural world.
While this is my operating assumption, I can't prove it. This is the distinction between positive atheism, which asserts that no God(s) exist, and negative atheism (my position), which is simply the lack of belief in such God(s).
Religion is just myth and superstition that hardens hearts and enslaves minds.
I agree that religion is myth and superstition, but not that it always hardens hearts and enslaves minds. Sometimes it does just the opposite. Humans are strange animals...
(2) the people who agree with the text?
I think that people who agree with the text are positive atheists who have a chip on their shoulder regarding religion. Really, what more could be inferred?
I agree with a good deal of your post, including this part.
As for the distinction between negative and positive atheism, one of my critiques of that Heinlein quotation is the use of "religionist." At least out of context, it can easily be taken to mean only those who adhere to what is conventionally considered a "religion." My experiences with Positive Atheists have shown me some remarkably tenacious (though annoying) examples of faith.
(My biggest reaction to the quotation is my skepticism that any intelligent person engaging with our world actually lives in the "cold light of reason" anyway as opposed to the "warmth" of faith, but I'll probably post more about this reaction later.)