Maybe it's a security issue? With some sites you might not want that information saved, perhaps.

Good call. Not thought about that. At least, it is shown to the user, and not hidden and recorded secretly.

Opera saves forms when possible. It also has instant backward navigation from cache. I guess it saves forms on cached pages.

Firefox seems to do this sometimes too. (e.g. I went backwards then forwards while typing this, and my text was retained.)