On modern browsers, you can fix that by using HTML5's replaceState function.
Do you happen to know then why popular sites like Facebook aren't using this approach to preserve infinite list position when going back? There probably is some big disadvantage i'm not seeing.
For another thing, it's a bit more complicated to implement (you need to have a valid URL for every little piece of every page on your site) for not a lot of benefit that most people would notice. In fact, if you don't do them really well, it might confuse people when their bookmarks don't take them to the start of the page. They're sort of the awkward middle child between permalinks to an index and permalinks to posts, and even if they were there people wouldn't want them as often as the other two. This would probably put it in the "Maybe get to this later" pile.
Not to say that I don't agree with your conclusions about infinite scroll -- I'd much prefer sensible pagination.
I'd like to point out, however, that FB already does some kind of similar trick with the URL hash when browsing photos: if you open someone's photo, it'll open in a modal dialog where you can navigate to other photos and this in-page navigation _will_ change the URL hash. If you go to that modified URL directly (or refresh the page) FB will show a different page (inside the person's "album") with that photo.
I'd reason that, for FB, a photo is more of a tangible thing than an item in your infinite stream, thus it deserves separate URLs when browsing the them.
Also, yes, i agree in that it could be perceived as a strange behaviour, but to me the current behaviour is also quite strange: if i'm scrolling down the FB stream, triggering a couple of stream sudden enlargements, then go somewhere else on that tab, and then go back to the FB stream, the browser will remember my last scroll position on that page and will try to scroll down to where i was, unfortunately that position does not exists when the page is loaded so it ends up scrolling to a seemingly random point in the middle of the stream (actually it's the bottom of the stream when the page loads, but that triggers a stream-append and then it's not the bottom anymore), not where i was before nor at the start of it (at least this happens in Firefox).