Suspecting that a book might be aimed at an American audience isn't actually a theory, and if you want to look at it as a theory it obviously can't be falsified by a single counter-example.
That was published by
which had been a fairly academic publisher.
"The Bell Curve" was neo-conservative propaganda, aimed at reactionary Americans.
Publishers are much more interested in making money than anything else. Academic presses do concentrate more on university libraries (and there are about 2000 of them in the US) than regular publishers, but they do seem to happy to exploit their academic status by publishing authors who want to look academic. Herrnstein actually was an academic, if obviously not a very good one, and while Charles Murray was described as a political scientist, he was more of right-wing rabble-rouser.