It's enabled by default, and therein lies the issue. Some of us want a shortcut for all the apps, others want to do things manually and only add a few. If you don't change the default, you can become inundated with shortcuts. This is especially true when you are setting things up for the first time, and install all the apps in your list in one shot.
You can easily change this default settings from Settings section. Just Open the menu » settings » general » you'll find an option that says "add icon to home screen." If that box is checked, an icon for every app you install will be added to your homescreen, in a seemingly random fashion. Uncheck it and they won't.