That's different, you talking about a trial account. You can play on a trial account without actually buying the game. Once that time expires you will be required to buy the game which usually also comes with the 30 days free. Typically (not sure about WoW as I've never played it) the trial accounts then are upgraded by you entering in the game key when you purchased the game. Some MMO's the trial game install is actually separate from the game install and require you to uninstall the trial client then install the actual game client.