It is a content script that calls the background.html. The content script only runs on Wikipedia/TVTropes domains. The browser action is just to open the WikiTrail page. This way you can access it from the toolbar instead of first visiting Wikipedia.
Edit: Ok, I understand what you mean. I'll try to make it a page action and post an update here.
I'm not sure if I'm willing to commit to a button that only serves as a bookmark. Other extensions handle this by using the options page for such a page. Button spam is a serious problem on Chrome, everyone wants to be a browser action when few actually offer meaningful features for such buttons. Xmarks is a great example of this. It's just a menu.
On another note you might want to submit this to /r/chrome subreddit on reddit.
http://code.google.com/chrome/extensions/pageAction.html
Button space on chrome is valuable real estate. Don't want zombies sucking up the space when not needed. Cool extension though.
I'd use it if you made it a page action.