No, Heinlein didn’t preach. What he did was be an American. Be an American so intensely that it was impossible for his work not to transmit it. The internal logic of the stories themselves led you to believe and think like an American.
I think that’s why the left hates him so much. At least that’s the only reason I can imagine that a writer who went out of his way to have minorities and women in starring roles in his books can be considered “racist, sexist, homophobic.” (Well that, and the fact that the left has heads full of rocks.)
In these days when the Betsy Ross flag is being transmogrified by leftist hate and intolerance into a “White Supremacy Symbol” (I ordered one for my yard, btw.) it’s impossible not to realize that what the left is trying to do is cut us off from our own history, our own symbols, and that which makes us uniquely American.