I personally would expect the back button to go back one slide, and would argue the opposite: that having the back button skip all the way to the previous web page is unintuitive and not "internet friendly". I mean, imagine if this slide deck were implemented using normal links: the fact that they are using some JavaScript-oriented feature to change the hash instead of changing the path shouldn't change the functionality, and implementing this without JavaScript would make it more clear that this is conceptually a page transition.
But how would you go forward in the first place? Not with the "Forward" button, because you haven't visited the page yet.
Instead, you press "Right" or "PgDn" or what have you. So I would expect the converse button ("Left" or "PgUp") to go back a slide as well (which it presumably does).
That leaves the browser's Back button to go back to the previous web page.
Why do people do this?