The iPhone app is free because it's their mobile answer to the browser based desktop version. The browser based Pandora IS your "free" version for the computer. Makes perfect sense.
You might want to try using something like Fluid to create a desktop Pandora app. I've thought about doing it for a while now, but I haven't gotten around to it yet. I'm not sure how well it will work, but it's worth a shot.
EDIT: Just threw it together this afternoon. Works like a charm. You can even make it run in the menubar where it only pops down if you click on it. This is a great way to passively listen to Pandora on a Mac and only access it when you need to change something.