Jarkko, you are not forced to buy anything to develop flash applications, just go to Adobe's site and download free SDK with AS compiler and use editor or IDE of your choice to write AS3 and MXML code.
Actually several days ago Adobe has contributed code (135 KLOC) from ActionScript Virtual Machine (AVM2) to Mozilla Foundation's Tamarin project which goal is to implement a high-performance, open source implementation of the ECMAScript 4th edition (ES4) lang specification.
That means in future we'll be able to share algorithms and libraries between browser with it's own DOM and Flash with it's media oriented DOM. It is a little step but, standardization is always good
Just try to forget about that "interactivity" (we aren't designers after all, right?) and look closer at Flash platform and opportunities that it gives to web developer, of course there are ugly things made with Flash, it is the cost of popularity.
About back button in Flash: this problem isn't problem for several years by now! There are hundreds of tutorials for begginer flash developers describing how to enable back/forward buttons and history in browser, but many of them are just to lazy to implement this. Flash is going to desktop along with traditional HTML and AJAX/JS, try googling Apollo.
So, I wish you only best experience with whatever technology you are using, especially with Flash