I'm going to have to agree with Justin on this one. I appreciate the instructions on how to make a mod for this.
The reason attending should be the default is that not only are you (the user) interested in your up coming events but usually you will attend the events you created. It's not a big deal, but is ideal to show more relevant content first. Another way to think of it is that typically people attend more events than the ones they create. This is because events typically have multiple people, hence making you less likely to be the event creator (unless you do this for a living).