For me, the issue of a reload/update (rather than a refresh from cache) when pressing the "Show the previous page" back button only occurred when I had the Safari Extension "LinkThing" installed and enabled. If I disable it, then it works (i.e. refreshes from cache); if I enable it, then it doesn't (i.e. it reloads/updates from the actual website).
I wanted some of the functionality from LinkThing such as removing the URL redirection from Google links and wanted the invisible (show only when hovering) status bar to show link addresses. I now get this functionality from the Safari extensions "Detox" and "Invisible Status Bar". I also have "SafariRestore" and "Open in Internet Explorer 1.0" (the latter from Parallels) and none of these have a problem.
So only certain extensions trigger the problem from Apple where going back reloads/updates the page. There was some reply in this thread that gave a technical reason for this.