Batman and Harry Potter are fantasy universes, and that encourages a more visible type of fandom. And those franchises have all kinds of "canon" minutia for nerds to geek out over. The F&F movies are ridiculously popular, as evidenced by their box office, the number of IMDb votes, and the fact that they've made 7 of the damn things.
I've only seen the second one, which I thought was mildly entertaining but not enough to keep me coming back. I don't care about the series, so this was the shortest episode of FS ever for me (intro, notes/filmspotting madness, close). But I don't deny the popularity of these movies, or the fact that people who I consider to have reasonably good taste seem to enjoy them. Suggesting that Adam & Josh are reviewing Furious 7 as a grab for a wider audience is ludicrous. And the idea that they're getting paid off by the studio is insane.