This is because with iOS4 and multitasking, an app never really quits. It just goes into the background and remembers exactly where you were.
This is similar to your desktop computer. If you put your web browser in the background to work on another document and then switch back to the browser window, it doesn't reset, it just picks up where you left off.
The "start page" preference is only triggered when the app is started, which now happens very rarely. We are exploring ways to get this to work again. Perhaps by keeping track of how long it has been, and if you resume the app after X minutes, it does the start page, otherwise it keeps you where you were.