Regarding not making the back button do anything in certain cases, I think part of the problem is the shitty infinite-scroll, endless toilet-paper UX that many modern apps use for pagination. It makes a lot of UX choices awkward.
Say you're building some Instagram or Pinterest type of App and you just have some kind of endless scroll to view content continuously. The user scrolls for 10 minutes seeing 100 pages worth of content on one root URL, and then clicks on another page and then goes back. What then? Do you put them at the bottom of the 100 pages on the original page after they hit that back button?
It's not super difficult to save say the last item you scrolled to and go back there when you reload the page from a back button press, but that specific UX just makes it awkward to know where you even are within a specific page. Maybe if people get lost when going backwards and that's the UX choice, it's not even worth bothering.
In the case of Reddit, probably when you click interesting thing your current url is saved to history and then you go forward, you go back the url is used to construct page again, but that url probably represents a concept like "Currently most popular posts under subject X" not what lifeplusplus was just looking at with particular offset etc.